I think everyone knows how I feel about the 501s by now.:thmbsp: I am also a big fan of tubes, especially amps. I even bought the 275mkIV. I usually listen to them in rotation of 1 week each just for fun. Lets see, the 501s are completely grain free, have a warm, refined midrange, have silky smooth highs that are non-aggressive and never fatiguing and bass that sounds just "right" to my ears, full of texture and definition. If that sounds like I am describing tubes, well, that's what they sound like, my 275mkIV has nothing on 501s in terms of tube advantage. At the same time the 501s also have at least 500w on tap for driving any speaker well and have better control. It's not like any other SS amp I've heard before other than McIntosh's own 402 which is similar. The 501s do have a slight advantage over the 402 though, to my ears mostly in terms of soundstage and focus. Not sure what tube amps can really offer you more of over the 501s either, maybe a touch more harmonic bloom but the 501s are not that far behind with that either.