I have honestly been wanting to learn exactly why this is true. I have read way too many home audio threads putting tube amps as god's gift to humanity while anything solid state is lesser. Since, I have been reading your posts for over 2 years now, Zane, can you either elaborate on this (actually, I don't think this site has the bandwidth for that

) or suggest some reading material?
There's really no fancy 1000 page novel on this matter, it's quite a bit more simplistic.
Simple logic-
Take away all the sound enhancing devices often included with amps- crossovers, eq's, etc.
You are left with an amp, which sole design and constructed purpose is to strengthen a signal.
Comparing amp to amp. There is nothing internal that has even a remote function of coloring the signal. No internal component in it's layout on the main board posesses this ability in it's manufactured purpose as an amplifier.
To make it more easy to grasp; I pose this question:
What part of an amplifier generates SQ?
Some people argue that Tube Amplifiers create an added "warmth" or second distortion to the sound. I simply call it a personal conditioning brought on by mis-information, resulting in the expectation that there is an audible difference when one is not really present.
THD in nearly all mobile amplifiers today is below 1% which has been found to be in-audible to human ears.
Even if the THD would become audible, it would not have the sound enhancing characteristics which are desirable in a stereo system. In fact, it would be quite the opposite. Distortion is what you would hear, thus minimalizing actual clarity. Good reason that most mobile amp makers keep the THD below 1% now isn't it? Coincidence? I don't think so.
I have administered and participated in several blind amplifier comparisons, and personally have never been able to discern any audible differences between the two amps, nor have any others who have tried.
I know that Josh (jlaine) from SIN has likely done the same test far more times than I have, with similar results. Thus we both share the same theory that a watt is a watt.
On a further note, I am a guitarist. Saying such, I am supposed to subscribe to the idea that tube amps sound better than solid state models.
Funny that I find my all tube driven 68' Fender Bassman to have no distinct audible warmth or advantage over my solid state Crate GX-60R.
Nothing complicated about it, an amplifier in design simply does not have the ability to do anything other than amplify. If it does, it's either broke, or it's not an amplifier.