ok, now were really beating the dead horse!!! (Im not trying to argue but just enjoy the interesting topics and hope someone gets something out of reading all of this)
some things that separate better sounding amps from worse ones:
Slew rate and damping factor
These are definitely measureable.
The tube amps that people hear the "warmth" is definitely not fiction; a tube produces harmonics that add to the sound. Granted it is "distortion" and changes the original but in a pleasing way; and very minute; it takes a very keen ear to hear the difference. In the recording or live audio process audio is purposely passed thru tube preamp stages just to restore what is lost in a/d and d/a processes, or when driving tubes hard the distortion produced is sometimes what an engineer is looking for on a particular track (the distortion does not sound like a solid state amp distorting, it is much more pleasing than that).
If you consider yourself an audiophile and listen to your system a lot and your favorite music often you can easily pick out the nuances that are less noticeable on other systems. The double blind listening test in the link put a bunch of people in a room and not everybody could notice the difference, but not everybody has the ear that it takes to notice; if you don't have a trained ear then it is not worth spending the money on something you wont notice the difference on; Also, the test must be instantaneous switching to really notice the difference because once it stops your memory isn't good enough to really notice the small nuances that could be changing. Its like an old lady here in Couth Florida buying a 6 cylinder brand new camry to drive it 25 miles per hour to the food store twice a week, rather than the 4 cylinder version or a used cheaper car. Chances are she will never even know the torque the car has to offer; Or another (more appropriate) example is a semi-blind person buying a brand new hdtv when they cant see the great picture the tv has to offer! Being "semi blind" they may be able to see the picture but not notice how much better it is. If your ear is not trained to or used to picking out the soft nuances then why pay for a $2500 sound system!!! On the flip side, if you can hear the difference, don't be fooled by a report that says to not explore the higher priced gear because there is no difference! If your on this website chances are you either already do have this ear or you find it all interesting and want to spend more time listening to high quality gear (and after listening often you will start to hear the nuances, its most noticeable when you listen on another system and realize the difference in the sound).
I for one have listened to different amplifiers and can easily hear different character in different brands or designs. Can I tell which is which, not necessarily (depending on which 2 are in question) but I can hear a difference and tell you what I like or dislike about each, or atleast what the difference is. It is hard, not huge difference but there are differences.
Hopefully in the >$600-700 range, theres not much to dislike and just slightly different character from one to the next.