IMHO AKG 701, 702 and sennheiser 598 are great headphones for piano, both replicate piano timbres well. Personally I prefer the 598, but the AKG is the better phone on paper in terms of slightly cleaner response, better imaging, wider soundstage, but for me that is not too important for just piano alone, and more a question of preference.
The AKG is more clinical and detailed and a bit colder brighter sounding than the HD 598, so it comes down to preference in the end of the day. They are both great phones that will serve/please most well. The advantage of the 598 however it is driven easily and rather less picky about sources you plug them into. The HD 600/650 is also a troublesome beast to be driven without an amp. Unless you own a CA 67 or above they have the grunt, pretty much anything else I heard no.
Interestingly I got the Fidelio X2 last week seeing you mentioned it. Unfortunately I can't test it out with my software pianos due to room works, but listened to many solo piano tracks and classical music with it over the week as well as recordings of software pianos.
Personally I think it is not the best for that IMHO since it is a very warm, a rather slightly recessed mid range. Yes, it has plenty detail all the same, but a somewhat U to V shaped out of box EQ. Super sound stage and very decent imaging, bass extension is among the best of open dynamics I heard to date.
However piano sounds have a tendency to disappear all a bit too easily compared the others and are very laid back and warm in that region, thus lacking that extra bit of impact/energy. When a piano sounds like it is a bit more closely miced I find it less of an issue and it can actually be quite nice then.
Personally I like a warm(ish) sound, but I even have my limits. Compared to the ones I mentioned above, the tonal characteristics and timbre of an acoustic piano do not come through that clearly on the X2 compared to the AKG 701 and 598, which are more forward in the mids to treble, except in the higher overtones where more of the treble region comes in, the X2 lifts up again. ( EQ can help a bit though but only got me so far).
The X2 is more expensive anyway and the way I see it you can pay less and get a clearer a bit less muddy piano sound with the others. It is still a great all-rounder though and you may well like it if that is the sort of signature you are after ( it is not traditionally flat response phone I would say, but a phone more designed to sound like good Hi Fi speakers in a good room ).
I would also add that it usually takes me a few weeks before I entirely acclimatise to a new sound signature before I feel at home with it, or know whether it is really for me or not in the long run ( however, for me at least, I doubt it will do for just solo piano ).
Since you like Pink floyd, Endless river sounds amazing on the X2
, the imaging and sound stage are stupendous for that type of music. Anything really classical string instruments, piano, I'd say not quite so much, still very pleasant though.
ath 50x very nice phone, very detailed, clean, crisp, great bass, mildly aggressive sound. Somewhat coloured though and rather narrow sound stage ( not too spacious sounding like many closed cans), but in fact I find it enjoyable to listen to many classical tracks and piano sounds on them. That said, I don't feel they replicate those sounds closely/accurately in terms of tone and timbre ( not too flat overall in the end of the day ).
Long time I heard a DT, never got on with them, treble troubles mainly, no EQ can solve that for me, a little bit too painful for me personally is my recollection.
For piano I'd stick with open designs if sound leakage is not an issue ..
At worst, save yourself long term ear damage with earbuds and higher volume levels
In the end of the day software and some stage pianos have quite a few adjustment you can make and customise EQ and sound a bit to pair it up with a headphone, that gives more scope to get it how you want, but not all pianos have that luxury and consoles are rather restrictive that way. In the end it is personal anyway. Sometimes it can just be luck of the draw what you like and appeals to you