It's been while since I made the thread, a past few months I have been testing a lot of headphones to find the one I like, and in the end I settle with Beyer DT 700 Pro X, after sometimes with HD 560s.
Using the 700X, I have to pull the volume on the 515 to 3/4 because it is that loud, and after trying these headphones, I found that M50x has a strange bug that would make the bass notes jumped in volume suddently, at first I thought my left hand put too much force on the keys or something, and had been trying to correct the technique hopelessly, after using these headphones, I am now confident about my left hand, and no sudden loud bass notes again!
Compare to the 560s, 700X is equal in all the range, being low impedance makes the 700X louder than 560s, only after trying these I could hear the sustained sounds of the CFX samples, before that, the M50x cutted the notes immediately (no decay) after releasing the keys which made me practiced with Reverb around 30-40% to counter it, I can clearly hear the decaying sounds very clearly with both new headphones at the default 10% Reverb of 515. Both 700DX and 560s sound too damned good but I live in a noisy neighborhood so in the end the 700DX fits better, the 560s is now for listening to music!
Sorry for my English, and hoping these findings would help someone in choosing a good headphone for P-515!