It is hard for say what are the best amps I've heard, given the variability of setups, rooms, other components, etc., but, I can say I've hear some nice sounds from systems that used certain amps. I've consistently heard nice sound from systems with Ayre electronics, so I suspect their amps are decent. I've heard two First Watt amps, one in my own system (J-2) and another in my friend's system (SIT). The SIT amp was actually his own build, based on schematics provided by First Watt. I've heard a couple of decent sounding systems with D'Agostino amps, so they are probably top contenders as far as my particular preferences go. I recently heard a couple different amps from the current lineup from Luxman, and they sounded good; I was particularly impressed with a low-powered integrated that seems to be quite nice for the money.
I've heard systems with top contenders, like Soulution and Constellation; the systems did not do anything for me, but, I cannot separate out the contribution of the amps, so I have no opinion about their sound.
A friend who is, like me, very much into the sound of lower-powered tube amps, said that he liked the sound from a system powered by Bridge Audio Labs (BALabo); that company is out of business and the amp was a six figure item.
