I bought a pair about a year ago to mate with Logan CLX's - which are very revealing speakers and have an impedance curve that can challenge some amps. I also have a Plinius SA250 Mk3 to compare the 501's with, which is one hell of a beast and Class A to boot.
My take on the 501's is that they have lots of drive, detailed, musical and appropriately emotional in tone. Easy to listen to - but I never feel that they are so smooth that I am missing something - in fact with the 501's I can hear way deep into the music. Also I disagree that they roll off the top octave, but that might be a synergy thing.
Very natural, beautiful build, cosmetically .... great audiojewllery!
There will always be something "better" but for me after many years on the audio upgrade highway,changing out these amps would be a low priority and I am sure anyone who buys them would not be upset.
Steve