perhaps it was only MY 901 that didnt sound good enough, as hifiman has always had QC problems over the years. but their background noise, as we have agreed in another audiophile chat group, is almost unbearable! unless u r using re600 in the balanced mode. and as for their ability to drive high impedence headphones, 240 is actually much better. but yes u have to use a much higher volume, but in terms of distortion, its almost non-existent! but 901, on the other hand, cant even drive their own he500. but they sound phenomenal on ak240. believe it or not, give it a try, i have tried many headphones, including hd650 240 is better by a long long shot!
wow - never have issues with background noise with my 901, even with low-impedance headphones/iems. never knew that to be an issue.