Hi FordPrefect, the kid, and all,
If we are speaking in a technical sense, tube amplifiers tend not to be "linear" as solid-state designs; that is, the frequency response isn't as smooth, so certain octaves in the musical spectrum may get slightly emphasized or (more commonly) slightly depressed, especially in the midrange. Virtually every solid-state amplifier these days delivers ruler-flat frequency response, especially if you switch out the tone control circuits which may introduce audible blips.
It is partly these non-smooth variations over the musical spectrum found in many tube amplifiers that account for the differences in sound quality heard in comparison tests. However, if a tube amplifier is properly engineered to yield linear response, these differences evaporate. I speak with some authority here, having done A/B double-blind tests using Macintosh amplifers along with solid-state amps. One of the other panel members was Ian G. Masters, whose quote from a current issue of Sound&Vision is cited by FordPrefect.
One enlightening demo illustrating the above was done by Bob Carver, who claimed he could make a solid-state amplifier that sounded exactly like tube amps. He did that, and to achieve it he altered the frequency response of the solid-state amplifier, adding a midrange slump in response (that adds a subjective sense of "depth" to recordings) followed by a lift in the upper octaves, to add a sense of "air" and "bloom" to the instruments, the latter both qualities adored by anti-science types like Harry Pearson, founder of The Absolute Sound.
There are also interactions of tube amplifier output stages with a speaker's impedance curve that may introduce quite audible errors in frequency response, and those will be peculiar to a given set of speakers, and will be different with another pair of speakers. Solid-state amplifiers have essentially 0 ohms output impedance, so these variations away from accurate response do not occur.
In short, tube amplifiers are inaccurate, non-linear devices for the most part. It may be fun for hobbyists, and those glowing orange filaments sure are purty, but accurate sound is not a virtue of tube technology.
Regards,
