In terms of sound, I initially tried routing the Pianoteq sound back to the Grantouch GT2 speakers, while it sounded quite thin and not satisfactory, it did added some feeling of vibration and feedback to back to the keys (then to my fingers) while I played, which I considered an important component of the piano playing experience, which will be missing if I routed the sound to an external speaker system.

The next step I tried was to "upgrade" the Grantouch GT2 speakers, and I ended up making speaker cabinets with the same dimensions to the original Grantouch GT2 speakers, but using a higher quality speaker driver (Jordan JX92s). It significantly improved the sound, but it is still not totally satisfying to me.

So I ended up routing the sound to both an external sound system, as well as back to the Grantouch GT2. With this, the overall sound quality is excellent, and I get some vibration and feedback from the piano to my hand as I played. I am quite satisfied with this setup, but I am not sure whether it is better than a good acoustic grand piano. Sometimes I still find the sound a bit too "hard". But I can say that there were times when I played an acoustic grand piano in different settings, where I thought "this piano is really bad, I much prefer to go back home and play Pianoteq controlled by the Yamaha Grantouch GT2!"

With respect to the velocity curve, I find that I have to press really, really hard to get 127 (almost impossible), and really, really soft to get 0. So I adjusted the velocity curve to "map" to 127 earlier.

My two cents...

I would also be interested in the answer to that question.  I have used a Yamaha GT2 for about 20 years.  It needed a new controller at one stage at a cost of about $400, but the action has remained in perfect condition.  The native Yamaha sound as it was then lacked a bit of character and when I discovered Pianoteq only a few years ago I connected the Grantouch MIDI to a laptop running Pianoteq and the output to the Grantouch Aux In with the internal volume turned down so the speakers carried only the Pianoteq sound.  The sound was rather “hollow” or “nasal”, quite unpleasant to listen to. 
(...)
I use Sennheiser HD599 headphones and play classical music daily, often for hours as I am now retired.  The Yamaha wooden keyboard is fantastic with a real grand piano touch and sensitivity.  The setup can all be concealed under the Grantouch lid using a laptop that folds out flat.  It is a total grand piano experience with headphones, or with the combined sounds of Grantouch and Pianoteq, but I would certainly like to get the Pianoteq speaker sound that others seem to be able to get.

Do you get the same "hollow" or "nasal" effect from the headphones too?
- if so, you are one of the ones who simply can't stand Pianoteq sound and based on my experience with some of them (including close friends), there is nothing you can do to fix it. Try Garritan instead if you use a compatible OS, which it's usually liked by those who dislike Pianoteq.
- if not, use other speakers, rather than the one inside the GT2 -- either find cheap ones on craigslist if you are in the USA (there are many gems that are sold for peanuts) or buy expensive monitors (often costing much more than a digital piano itself, sigh)

I now find that a very much better experience is listening to the Grantouch and Pianoteq sound together, either with Pianoteq through the Grantouch speakers, or through another good quality sound system.  I have explored all the tweaks that I can find on Pianoteq but to no avail.  I have written about this boringly on another thread and received kind and helpful advice, but the problem remains.

I use Sennheiser HD599 headphones and play classical music daily, often for hours as I am now retired.  The Yamaha wooden keyboard is fantastic with a real grand piano touch and sensitivity.  The setup can all be concealed under the Grantouch lid using a laptop that folds out flat.  It is a total grand piano experience with headphones, or with the combined sounds of Grantouch and Pianoteq, but I would certainly like to get the Pianoteq speaker sound that others seem to be able to get.

If U asking questions like this : ..." There are quite affordable quality midicontrollers with good velocity sensibility, like M-Audio Keystation Pro 88, with hammer action. They are only 400 dollar. ..." - it means U are not piano player . With this M-Audio U will spoil Your hands until become musician , my friends had this Sh.... , and after few days sold , twice less price , because nobody needs to have not useable midi controllers , which after 1 year using keys of these kind keyboard become like waves in the ocean . But ... if U are   playing 2 hours per week on this keyboard , and U enjoy  how nice it looks in your home studio this kind of controllers it's O.K. I know that in most countries people first look , how it looks , how much it cost , after what is profit from that and last how U feel Yourself. When U are playing per day 8-10 hours , needs to have good instrument , good feeling instrument , after U have feedback  between Your ears - mind and hands , here is secret of good playing , hearing and creating something good , but not between $400 and nice looking ,  here U will find only image or immitation , of real quality of feel , sound & creation .
Not forget , that there is no miracles like  " ..quite affordable quality midicontrollers ... for only 400 dollar ..." U can ask same , why to play Stainway Grand Piano ( for & 200000 if U can play Upright Puiano for 30000 , what is different ... answer,- maybe U never tried this, try , close Your eyes and listen carefully , if U not hear different between them  , ok buy Yamaha PSR series keyboards , and enjoy .... few weeks .
On yamaha GT2 Midi controllers are laser beams , On other more expencive Midi controllers there are more MIDI velocity steps , this is big different from cheap controllers , because some of them have velocity , for egz only 3 steps , for egz. like  p - 53 . f - 80 , ff - 127 , this only immitation of velocities , but not real velo in 127 steps ... and some musicians claime , that 127 is not enough , but others not hear different between 3 steps velocity and 127 ... What to say .... sorry .....
First impression from keyboard this is good or not is , that when U are playing , after some time U not feel bored , and U always need to play . That is with Pianoteq , U are playing only piano and not bored , if U have a good quality controller , again Your hands not be tired ,  it's same like to find what different between cheap , and expencive car or gum and real women , try .... and U like it ....

PLEASE tell me how you altered the circuit board to make the AUX input not so sensitive.

Yes we tryed Yamaha GrandTouch with Pianoteq 2.1 . We connect our PC soundcard output thru GrandTouch build-in speakers . It sounds very realistic and piano quality ...

Hi Voxas, congrats, it looks great!
i have a Grantouch GT1 and would love to do the same.

Did you plug the 4 built-in speakers directly to your soundcard, without using their electronics?

I plan to put a mini-pc inside my GT1, what soundcard would you recommend?

For the MIDI interface, did you use the external one or plugged somewhere on the Grantouch motherboard?

Thanks!
Arnaud

   Well, if the sensor of Roland digital pianos could be placed in a piano actions I would be happy.

     I was trying to get contact with a piano manufaturar to try convince them to create a midi controller using real piano actions. I really believe it's possible to make a affordable model with great touch feeling.

       What exactly do you mean by balancedin weight? Is it about trebble be less heavy than bass? My Roland have some difference about heavy from bass and trebble.
     
     Is after-touch the same as scapments, reffering about technical nomenclature for digital pianos?

     Thank you for the links and all informations.

                  Thats all ... amen

I never played in Keystation 88 Pro. Just got some reviews on web. I play in a Roland F-90, 88 keys progressive hammer action digital piano.

   Bu I think would be very interesting to develope a Midi controler using real piano actions and keyboard.  I hope someone develope good sensor for affordable price.

  There are quite affordable quality midicontrollers with good velocity sensibility, like M-Audio Keystation Pro 88, with hammer action. They are only 400 dollar.  I supose it have some few good sensor, since it's a hammer action model, and based on reviews I got on web.

   So why any piano manufacturer built a real piano action in wood, and just add those kind of sensors used in affordable Midi controllers???   So we would have a real hammer action system Midi controller, exactly like the action of a grand piano.

   I don't know how is the sensor of most stage digital pianos. But the manufacturers could add it in near the keys or along the hammer's arm. There must be a way to produce a affortable real grand action in wood Midi controller.

    Does anybody have picture or diagram showing were and how the sensors are positioned in a digital stage piano with hammer action?