I agree alot with what Jim2 is saying. I think that much of what people perceive as "tube sound" is really "output transformer sound", since it is the output transformers that are generally creating the rolloff and bloat that some people mistake for "tube sound".
Regarding tube equipment being a "gimmick", it was the original form of audio amplification and was around long before the transistor was invented, so I don't think it can be called a "gimmick". I am of the opinion that properly made tube gear can be the very best sounding equipment possible. There are drastic differences in the sound quality of various tube amplifiers, just as there are drastic differences in the sound quality of various transistor amplifiers. The best of either technology can be very good indeed. At this time, and including all of the many pieces of gear that I've heard so far, I prefer the sound quality of the best tube gear.
Regarding tube equipment being a "gimmick", it was the original form of audio amplification and was around long before the transistor was invented, so I don't think it can be called a "gimmick". I am of the opinion that properly made tube gear can be the very best sounding equipment possible. There are drastic differences in the sound quality of various tube amplifiers, just as there are drastic differences in the sound quality of various transistor amplifiers. The best of either technology can be very good indeed. At this time, and including all of the many pieces of gear that I've heard so far, I prefer the sound quality of the best tube gear.