Maybe I just don't get you because english is not my native language ...

It's not your English (which is excellent of course).  Very few of us, even (or perhaps especially) among native speakers, understand Ra's often-times belligerent ravings.

Anyway let me extend a long overdue welcome to you at this forum now.

(Some of your ideas suggested as improvements to me are very interesting.)

Haven’t you already been convinced of the fact that you can apply equalization to low (low pass and low cut) and high (high pass and high cut) frequencies of the spectrum and that to the exclusion of any mids?

No, I haven't.
For sure I can eq some low frequencies and this will affect only bass notes beacause all other notes don't have bass frequencies. But if I eq high frequencies this will affect all notes because all notes have some high spectrum.

Maybe I just don't get you because english is not my native language, but I think you're mixing some things up.
EQUALIZER will act on the spectrum of a note played, the frequencies influenced by the EQ are unrelated to the note's pitch.
EQ3 will act on the spectrum of the whole instrument according to where it sits in the signal chain even on the reverb.
Overtones will act on the spectrum of a note too, the frequency influencd by the overtones are always related to the note's pitch (fundamental).
Overtones and EQ cannot compensate each other except for one specific note.

By the way a "dull" sound usually is lacking some higher frequencies, but the problem goes beyond only frequencies. The attack of a sound is also very important.

Well, I guess I'm very lucky that not many people did come to the same conclusion so far.

My conclusion at this point is somewhat different. I think either I still haven't found the optimum settings to make Pianoteq Standard sound like I want or I might just need Pro for it. And even if it already is a good product, there are always possibilities for enhancements in functionality and sound, which doesn't mean it is faulty. If a sample library doesn't have unacorda samples, it's not faulty either, it's just lacking unacorda samples.
A big thank you to all the people who helped me with their tips and tricks in setting up Pianoteq so far.

I'm venturing to say some are very negligible to any human ear.

The bottom line is; if inside the EQUALIZER pane you make a band pass of mids just to raise them and compensate at their overtones occurring in highs, only offset by your high shelf beginning wherever you should decide, you like any skillful audio engineer use PIANOTEQ as he normally would use equalization on an acoustic.

PIANOTEQ presents precisely problems no more than an acoustic piano does to the engineer certainly in the recording session!

The conclusion I draw right now is the seemingly overwhelming problems faced by the original poster at this thread are more a result from his inexperience as an engineer and software end user than somehow faulty piano software or any recently discovered ineffectiveness really resurfaced but that's always been obviously inherit in accepted industry standard equalizers and attributed to them.  That only just now has been brought thankfully to anybody's attention.

Although, numerous piano recordings might have been made unbeknownst by people (professionals) beforetime!

Haven’t you already been convinced of the fact that you can apply equalization to low (low pass and low cut) and high (high pass and high cut) frequencies of the spectrum and that to the exclusion of any mids?

Spectrum, yes.  Notes, no.  If you apply EQ to high frequencies, it will affect the timbre of mid range and low notes as well, because it's affecting the overtones of those notes.  It only takes a minute to adjust the EQ and hear the difference, the effect is pretty obvious if you make some big adjustments.

Now only the bass is too sharp, too percussive.
Back to the beginning, I can get nice mids and a nice bass, but not both at the same time. But maybe I have more luck adjusting the bass, than I had with the middle.

To me your statements are very argumentative.  Haven’t you already been convinced of the fact that you can apply equalization to low (low pass and low cut) and high (high pass and high cut) frequencies of the spectrum and that to the exclusion of any mids?

As mentioned earlier by someone else, your cue (Q) factor once adjusted may allow you to focus further upon only the mids.  Which are within your reach since you can always add another equalizer, giving a total of five (5) or even as many as six ultimately (6) if you so choose to fill EFFECTS slots with only the EQ3s.

When you depress and hold the shift key on your QWERTY keyboard as you adjust one of the points of any Q on the interface that becomes affected proportionally.  Which allows only specific frequencies of course targeted.

I’m saying, dude, anything is of course possible!  However to someone else, Philippe Guillaume has indicated specifically Dynamics lets anybody get essentially that same velocity effect (perhaps the same exact possible effect) from a combination of it and Volume adjusted within PIANOTEQ PRO.

Clearly, he has...

I understand as a musician myself, though a drummer, you’ve your own way an artist is to express himself indeed irregardless of the workmanship or even craftsmanship of others.  Man, just like you I was a newbie once.  Then, I really felt myself entirely overwhelmed.  Now I admit I was slow to criticize...   Ten years indeed a span longer than the week length you mention as an end user, I waited before I made my first post at this forum.

So, now I’m saying and probably will again say about PIANOTEQ, it ain’t all going to come just the way you want to you overnight.

Look at Philippe Guillaume; do a search if you must.  He started as a piano tuner, before a lot of his ideas of a PIANOTEQ software finally materialized. 

Today, if your own posts show anything it is that that software definitely is within your grasp, unmistakably.

Is note-off velocity 127 really the best choice? It means that the virtual damper muffles the virtual strings always as fast as possible. Damping is often associated with a "dull" sound, hmm.

Testwise I changed it to constant 64 (a horizontal line in the calibration assistant) and the note "sings" more and appears to be less "dull" at the same time. Maybe it makes sense to change this constant to a personal preference. Eventually the fast damping with 127 is a bit overdone in some situations.

That's a very good tip. I tried it and it makes the sound more realistic for average playing, only staccato is now worse. I guess I'll have to upgrade my trusty keyboard if I stick with Pianoteq.

I also switched off the limiter as suggested in another thread and although I couldn't experience bad noise from the limiter before, it also adds a little realism for playing. The limiter (or a separate compressor) is essential for mixing/recording, but just for me playing it feels better without.

As I said some posts above, very early I discovered the huge influence of the velocity curve on the whole instrument. I had found a curve, with which I played now for over one week. But whatever I did, I couldn't come to good sounding mid register. So today I went a few steps back and started over with a new (more loud) velocity curve.
Immediately the mid register sounds much better, the high register doesn't sound too bad and I can adjust it.
Now only the bass is too sharp, too percussive.
Back to the beginning, I can get nice mids and a nice bass, but not both at the same time. But maybe I have more luck adjusting the bass, than I had with the middle.

Can the velocity curve be adjusted on a note-by-note basis in pro? Maybe that would make a good feature request. For ease of use I'd say a per note velocity correction curve would be a good thing, maybe apply some kind of power-law curve to "bend" the velocity curve for each note individually.

Is note-off velocity 127 really the best choice? It means that the virtual damper muffles the virtual strings always as fast as possible. Damping is often associated with a "dull" sound, hmm.

Testwise I changed it to constant 64 (a horizontal line in the calibration assistant) and the note "sings" more and appears to be less "dull" at the same time. Maybe it makes sense to change this constant to a personal preference. Eventually the fast damping with 127 is a bit overdone in some situations.

But a volume velocity curve would be very handy.  88 key volume control does the same, but neither the stage nor standard has it.

There is no volume velocity curve, but the dynamics lets you control the volume variation between ppp and fff. And of course the velocity curve itself acts on the volume, though on the timbre too.

Volume per se can be edited note per note in Pro and Standard: simply double click on the volume slider.

If you're arguing an upcoming standard version that could offer individual note Spectrum profile adjustments will be an improvement simply because of the note adjustments over what's currently standard, I have to disagrees with you there...

No, I also think that from a marketing point of view the three products are well thought out. I was hoping for a model improvement which doesn't have such unwanted resonances.

Both, of course. I started to play with Pianoteq and at first I was very disapointed. However I've read what I could find in this forum and think I've made some progres. Then I came to a point where I needed help, so I asked and I got many very helpful answers. I tried all of your suggestions. The more I play with Pianoteq, the more I get it to where I want. But also the more I play with Pianoteq, the more I find some things I don't want. So I pass them on as a feedback. Also I tell what I've done so far, so maybe it is helpful for some other people later.
Many times I've read that there are Pianoteq lovers and haters and nothing in between. I'm in between. I love how Pianoteq feels, I love how Pianoteq reacts, but I'm still not happy with the sound. I've spent an hour every day to optimize the sound and I'm thankful for all your ideas. I still wish I get it to the point where I can say: yes, I want to hear this a few hours every day.

I've tried to use the different EQs but this doesn't give me the wanted result. I don't have a problem with the overall sound, just with the sound of the middle keys.

When personally I may shy away from any Spectrum profile adjustment, I left my comfort zone and raised a third (3rd), fourth (4th) and fifth (5th) slider with the middle one the fourth (4th) adjusted the most.  In addition Strike point (a bit left) I moved along with a Soundboard Cutoff (a little right). 

Now my result is a bright middle register (as far as I can tell).  Any possible brightness that might occurr in the higher notes as also a result is completely avoidable whenever I compensate by Res Eq and Res Dur adjustments made in the EQUALIZER pane.

If you're arguing an upcoming standard version that could offer individual note Spectrum profile adjustments will be an improvement simply because of the note adjustments over what's currently standard, I have to disagrees with you there er I have to disagree with you there...

As already a PIANOTEQ PRO end user, I feel MODARTT will do a disservice to current PRO users, if it extends the feature of the PRO version (which other people bought) to PIANOTEQ Standard, at its standard price.

Now how randomness might jive with calculations, that's maybe a topic for some debate.  I maybe have to rethink my Random usage at this forum.  Let me clarify, it's basically theoretical; I'm using it as an adjective!  Although I've used it at another thread when I suggested my idea (that is) pseudo high resolution MIDI as a means possibly to compensate for the limitations of entry level keyboards limited to an unnatural 128 velocities.

However, many get the results they want and are enthusiastic enough to post about them.

You have to keep in mind, PIANOTEQ PRO —unlike PIANOTEQ Standard— has been marketed from MODARTT primarily to target a number of end users including those whose demands likely a lot similar to yours and mine do require the note edit functionality, specifically.

So Cogito, I want to know if you're merely asking for help, or have just been simply passing judgement...

Now about multiple mono mic outputs equalized individually, I've already posted my other suggestion at this forum.  That suggests perhaps if PIANOTEQ were to feature a selection of modeled preamps connected to the modeled mics selection, end users may have a roundabout way to equalized left, right, mid, and side frequencies independently of the other equalizers to the mic input available in the standalone, presently.

Since prior to any advent of this digital age, widely received piano recordings were made largely with microphones connected to preamps, I see no reason to dismiss an apparent usefulness or long accepted staple such as a preamp, immediately.  I'm hesitant to conclude preamps have finally been rendered completely obsolete.  They especially were important to the engineers whose mixes which involved them are today still very much enjoyed.  Some even got awards from a clearly recognizable institution (NARAS).

I just rechecked the Toneboosters website.  It seems to get a binaural effect possibly more accurate than that offered inside PIANOTEQ

It's not a binaural effekt, it is two microphones in an artificial head. So actually they should give the best and most authentic sound for listening with headphones. Unfortunately they either don't or I just don't like that sound.

That wasn't the solution. Inverting the polarity only makes sense when placing some mics under the soundboard and some above.
I used a RTA to find exactly the ringing frequency and reduced only that one in the main EQ. I don't knwo why it is there, one can even hear it loud at the factory presets of the Steinway D. It's at 2960Hz.

My experiments now have brought me to a setup with three mics, one cardio at the left side of the music stand one fig-8 at the right side and an additional fig-8 at the very top corner of the lid for some more brilliance.
I'm still not where I want. For the players position I still have a very poor stereo impression, although much better than with the binaural mode. Yes, I've seen the stereo width slider.
And although the mid register sounds much better than at my first tries, it still is too dull. Also I find that the action noises are too loud at the treble and getting more soft the lower you get and that isn't only a masking effect.
I guess to bring this any further I'd have to output all mics separately and EQ them individually later.
Worst of all I found out, that inside the audio host I run into CPU performance problems which I didn't had with the standalone version.

Gentlemen, the journey continous.

I've tested Morphit, like Amen Ptah Ra suggested. It definately is a nice tool, although it is nothing more than a EQ flattening out the response curve of the headphones. The good thing about it is that I don't have to (and actually can't) adjust too much. Set the headphones and you're done. It makes my AKG less analytical, but that has been the reason I bought it. It acts like magnifying glass and I can hear all the details. With Morphit the sound is more distant, which is sometimes not so bad either. I think for finding the right sound I'll go on with its demo and and decide maybe later.

The big change with Morphit is, that it comes as a plugin and so far I used Pianoteq in the standalone version. So to bring both together I had to switch to an audio host. Unfortunately I lost all my adjustments two times after the demo time has run out, still have to figure out why it doesn't come up again like the standalone version does.

I just rechecked the Toneboosters website.  It seems to get a binaural effect possibly more accurate than that offered inside PIANOTEQ, you may have to use the Toneboosters Isone plugin, either in conjunction with Morphit or otherwise alone by itself.  Anyway it’s likely included within the download you just got.  Although, it might just come as a free download with Bus Tools 3 also from Toneboosters: https://www.toneboosters.com/tb_bustools_v3.html.

Admittedly, I do have to read now a lot more thoroughly than I’d previously.

PIANOTEQ permits a flipped phase at each microphone.  That is probably a single solution to your entire problem...

Fresh ears and some perseverance and you'll find your sound.

Try a right-click on "Unison width" handle - that reveals the "Unison balance" tool - I find + or - around 0.2 really useful as starting points.

That balance I think of as secondary to width but I'll usually try to remember to adjust it each time I alter width. (sometimes, I'll also try with and without, or raise/lower delay in Effects section at this time too, if it's activated - and other aspects of course, like Direct sound duration and so on).

For example, Unison balance can make nearly satisfactory unison less 'swimming' whilst more realistic, without inducing even marginal tuning issues (like "Condition" slider might) - or enhance body - there's no right or wrong but a matter of trial and error to our personal tastes.

Ver 7 will no doubt address many things we all like to see improved - from engine through all instruments. The recent update to strings (in Karsten collection so far) is no-doubt coming to all pianos - it's going to be wildly improved IMO.

Esp. agree with what you say here..

It's good to appreciate that early on. Over time we realize that the defaults for many items are so very rational and our sweetening of things can be way more subtle than we might begin with - I am glad to break bounds of the settings to brute some discoveries - but going back to defaults, esp. with fresh ears is really key to getting a good preset edits we can enjoy over a long time-frame.

Really glad you're making headway.