Sennheiser HD 598

What impendance are the headphones? If it's a high impendance, you might need a headphone amplifier.

Now check the impendance of the headphone port of the Bose speakers... Perhaps there's an impendance mismatch.

I just talked with the Bose support and for company standard they don't release such technical data so I don't know what is the impedance of my Bose Companion Series II speakers.

Perhaps if I connect directly the Sennheiser headphone to the jack output of my iMac should be useful ?

Yep, try plugging the cans in your iMac as well, see what happens.

I'm just watching that Pianoteq has the "Output" setting into the Devices tab in Options window, this point me to make this consideration: if I buy a headphone amplifier (and/or an external sound card, I'm a bit confused!, sorry) that I can connect to my iMac via USB should I use it to hear the sound of the Pianoteq through my HD 598, is it right ?

This way I can bypass the Bose Companion speaker that will remain available to be used by other apps on my iMac and also I shouldn't do in the future the switch off/on of the headphone's jack on the front of the Bose Companion speaker, is it right ?

I have the same headphones and computer as you

I just plug them directly into my iMac and if I set it max volume, I would blow out my ears.

I would avoid using the Bose headphone port because it just introduces latency and low quality circuits into the mix. (Bose tends to over-charge for mediocre speakers)

The HD598's are at the cusp of needing and not-needing a headphone amp. They'll benefit (particularly in the low end) if you have one, but you don't need one. (Anything from FiiOs is good )

EDIT: Side note, if you just got them, you'll need to break them in before they really shine....play music through them at a medium volume for 24-48 hours

EDIT2: Second side note ..... If you ever notice one side sounding rattle-y, then pop off the foam and dust shield (google it, its easy) and you'll probably find a hair in it, carefully remove it with tweezers and problem fixed.

If at any point you hit a low fortissimo, or listen to music with low frequencies and you notice a sound like it's shaking/rattling in the headphone. It's usually caused by a hair interfering with the driver, I had that happen a few weeks after I bought my headphones, and thought that I blew them......I was relieved when I found the hair

This one could be ok ?

D1-24-Bit-DAC

I'd avoid Terratec, they've kinda sorta abandoned the pro audio world. Drivers are not au pair with other more prominent vendors.

Have you used the D1 with Pianoteq? I tried one a few years ago and thought it had too much latency. But it's possible the latency was coming from somewhere else. I did however get better results (for whatever reason) using an Apogee One, even though it's sampling rate is around 45 kHz.

How to know what is the rightest headphone??? The one with more bass, or the one with more trebble?

What about get a real piano, record it, and compare the sound of the recordind listening with a headphone and listening to the real piano?

The problem with that is that the microphones will change the sound too, so you don't know whether it's the mics or the headphones making the difference.

All I can tell you is that Rachel is correct. When recording and playing sound, the result is always the 'real' signal modified by a filter function. Mathematically this is called a 'convolution'. Thus, the result is:

'real' signal (X) microphone response (X) AD converter response (X) DA converter response (X) speaker response.

This is always true and will never change; it's basic physics. Ideally the response functions are unity functions, but they are not. Even the AD/DA converter response functions are not unity functions, though they introduce the least distortion. Every physicist will tell you that if you measure something, you will always get a result that is heavily influenced by your measurement and playback equipment... and microphones and speakers are just that.

About the hearing tests: sound is not absolute. The (deci-)bel is a relative unit. Thus, the position of 0dB is arbitrary from a physics standpoint. It is usually chosen to identify the threshold of human hearing, and it is absolutely to be expected that measurements from 70 years ago maybe did not get that point exactly right.

As an electronics engineer I can tell you, why  on most power amplifiers connecting a low-impedance headphone leads to bad results: most of them have a resistor of a few hundred ohms in series with the output signal, in order to protect against shortcircuits when inserting the phones jack.
This is no problem with high-Z phones ( 300 - 600 ohms), but with low-z types ( 32-50 ohms) this leads to attenuations of some 20 dB.

Itis always wise to read the specs of a headphone amp. many of them deliver only some 50 milliwatts; for somewhat more headroom one needs output levels in the order of 200 milliwatts ( into 32 ohms).

I use the AKG K550 together with the Fiio E11k, which is very reasonably priced.