I'm not doing a complete scientific geek review / comparison. This is just my own experience with high quality Beyer and Sony headphones:

I'm a long-ago lapsed professional classical pianist and I want Pianoteq (with my Kawai NV10) to emulate a moderate quality professional-level acoustic piano in sound and feel. I'm therefor only using a few, carefully selected presets that can do this well. I'm not interested in weird / wacky / heritage / wonderful.

The Audio from my PC goes out to a SoundBlaster box which drives headphones and speakers etc.

I've been using BeyerDynamic DT900 Pro X (open-backed) headphones for about 3 years now. The first pair cost me £300 but the price is now down to around £200. They are among the top recommended headphones by all the gurus. They are superb - the difference between them and yer average 50 quid headphones is HUGE. They give a realistic, deeply wholesome sound. I thoroughly recommend them. Until recently, I couldn't believe that paying more could give me anything noticably better.

BUT..

I know the Beyer ones have frequency boosting and compensation etc. built in and it occurred to me that Pianoteq generates exactly the sound it wants you to hear - you probably don't need frequency-fiddling at all. I therefore bought a pair of Sony MDR-MV1 (open-backed) headphones (circa £350). These are supposed to be frequency-flat. OMG. The improvement is really, really noticable. I was always aware of a certain muddiness / lack of clarity in the bass with the Beyer ones; I used to put this down to Pianoteq being a modelled sound and the synythesis not being perfect etc. The Sony 'phones have disproved this and confirmed my suspicion: You don't need any equalisation in the headphones. They make the entire piano range equal and smooth; the muddiness is gone, no more bright treble spots, no more thunder-rumbling bass .. at least, with the presets I use.

Moral: If you can afford the extra beer-tokens for the Sony MDR-MV1 then go for it.

Update:

Replying to my own post here:
BTW. The Sony MDR-MV1 headphones are 24 Ohms impedance but the Soundblaster X3 says it only drives 32 Ohm .. 600 Ohm.
This should be fine and I have had no problems; you just have to remember to turn the volume down a bit to compensate. I have ordered an 80 Ohm impedance adapter/converter plug and I will try it but it probably isn't necessary.

I'm a long-ago lapsed professional classical pianist and I want Pianoteq (with my Kawai NV10) to emulate a moderate quality professional-level acoustic piano in sound and feel. I'm therefor only using a few, carefully selected presets that can do this well. I'm not interested in weird / wacky / heritage / wonderful.

The Audio from my PC goes out to a SoundBlaster box which drives headphones and speakers etc.

I've been using BeyerDynamic DT900 Pro X (open-backed) headphones for about 3 years now. The first pair cost me £300 but the price is now down to around £200. They are among the top recommended headphones by all the gurus. They are superb - the difference between them and yer average 50 quid headphones is HUGE. They give a realistic, deeply wholesome sound. I thoroughly recommend them. Until recently, I couldn't believe that paying more could give me anything noticably better.

BUT..

I know the Beyer ones have frequency boosting and compensation etc. built in and it occurred to me that Pianoteq generates exactly the sound it wants you to hear - you probably don't need frequency-fiddling at all. I therefore bought a pair of Sony MDR-MV1 (open-backed) headphones (circa £350). These are supposed to be frequency-flat. OMG. The improvement is really, really noticable. I was always aware of a certain muddiness / lack of clarity in the bass with the Beyer ones; I used to put this down to Pianoteq being a modelled sound and the synythesis not being perfect etc. The Sony 'phones have disproved this and confirmed my suspicion: You don't need any equalisation in the headphones. They make the entire piano range equal and smooth; the muddiness is gone, no more bright treble spots, no more thunder-rumbling bass .. at least, with the presets I use.

Moral: If you can afford the extra beer-tokens for the Sony MDR-MV1 then go for it.