Well, and this may not be 100% correct cuz, hey, I'm a box guy and not a circuit guru.
IMO a Tube amp has a few common characteristics:
1) They have pretty low damping factor - especially in the bottom end.
2) They clip much more gradually and softer than a transistor amp.
3) They supposedly generate even order harmonics when they break up and transistor amps create odd order harmonics.
4) They look horny.
1,2, and 3 mean they sound 'softer' or more natural than a transistor amp. They may not be as accurate or 'true' as a transistor amp for the same reasons.
Most people like the sound of a tube amp because they can be easier on the ears thanks to 2 and 3. Bass can be kinda sloppy cuz of 1 but that just means careful speaker selection.
#4 is the main reason I want one though. [img]graemlins/thumb.gif[/img] [img]firstname.lastname@example.org[/img] [img]graemlins/headbang.gif[/img]