I've been testing many headphones recently for using with my yamaha NU1
testing with nu1 sounds as well as vintage II / emotionnal piano virtual instruments.
here are all of them in order of preference
1.sennheiser hd280 pro
4.shure SHR 840
5.beyerdynamic DT770 pro
1.the good old ugly sennhseiser HD280 is WAY above the rest in my opinion.
it is nearly ideal for piano use. the reason I like it so much is because the sound is extremely clear. I would actually hate to listen to music through it as the sound is kind of thin, it clearly accentuates treble instead of bass... But plugged in the piano this is a different story. notes are clear through the whole keyboard range and harmonics are not muted like other headphones. with just a litle more bass it would be better though, it lacks a little punch in the lowend. also for closed headphones the isolation is not very good which is a concern with a NU1/avantgrand or silent piano where you will hear hammers while playing piano/pianissimo, especially with higher notes.
choosing between the other headphones is more a matter of opinion, the difference is more subtle. despite being for most of them considered very neutral headphones (some being excellent for studio mixing) they all accentuate bass much more than senn280. you wouldn't tell by listening to music but it is just obvious plugged in a digital piano. I wouldn't buy any of them as sound clarity is my priority.
2.I ranked sennheiser HD380 second because it is the closest sounding to 280 with more bass, but it doesn't have its clarity. isolation is much better. less confortable.
3. HD25 is very neutral sounding which is excellent for piano, but a step lower in terms of clarity.isolation is excellent but it is very uncofortable to me. the one I found it to be the most versatile, it is very good for music listening and decent for piano playing.
4. shure 840
too much low-end. best isolation still confortable (feels much better than hd 380). If I could put the sennheiser280 electronics in this headset it would be near perfection. couldn't find how to disassemble it properly though ..
5. beyerdynamic DT770pro
the most confortable with AKGs and HD600, a closed headphones that sounds very open. very good sound actually but not appropriate for piano in my opinion.
6.7.8. akg 242/akg k701/sennheiser HD600.
these are my favorite for music listening and least favorite for piano playing.
Being open headphones they are not suitable to my piano. I guess open headphones should be avoided for stage use as well.
also these are the least neutral sounding ones, doesn't work at all with DP in my opinion. night & day compared to my first choice despite being more expensive.I think headphones for digital piano should be tried with a digital piano and never through any hifi setup even listening to piano music. the difference is huge.
for this very specific use, paying more won't give best results, it seemed to me hifi headphones are inapropriate, neutral studio ones are the only one to consider.
just like your piano would sound way better through studio monitors than any expensive hifi setup
hope this helped despite my approximative english