I suggest that the idea that tube circuits are inherantly distorted or warm has been perpetuated by companies marketing tube toys.
There are lots of great tube amplification circuits that are splendidly clean. Solid state audio circuits were developed for economy of manufacture and profit. Of course solid state had lots of other more pragmatic uses... the audio was just a spin off.
The U.S. Navy still uses tubes in many of their hi powered circuits because they think its misson critical.
By the earlie 1980's Tube gear became difficult to locate and purchase because it was so much more expensive than solid state mass production. Then came the nineties when musical instrument companies found sneaky ways to market low grade tube circuits to an audience that was willing to buy in to the idea that tubes inherantly sound distorted. A few hi-fi companies tried to do it but found out that hi-fi listeners actually demand good circuits. Luckily if you want and can afford real tube musical instrument or hi-fi gear there are more choices than ever... they are just harder to locate than in the past. It's still being made the best way people know how for those who want to experience it firsthand.
Remember when Jimi asked are you experienced? have you ever been experienced?
well... I have. :-)
After a life time of studying and experiencing the sound of both instrument and hi-fi tube amps I have come to understand that the one "truism" regarding tubes is the remarkably fast transient response. You can measure THD in a tube circuit and you can look at slew rates in Solid State or you can just listen to both and hear the difference. A real tube amp is more responsive to any input. It doesn't matter if it's a electric guitar, electric bass, a nice microphone or a recording of a soprano vocalist.
Personally I think the fast transients of tube circuits provide the "cleanest" musical sound (as opposed to a sterile sound) and think the Fender Twin, and Ampeg SVT are fantastic examples of clean musical instrument amps. I also think it's this basic quality of quick response that allows a tube circuit to sound so speacial no matter what gain structure you arrive at.
Any discucssion of inherant distortion should really be focused on the gain structure of the circuit. That's the distortion every one is talking about... you'd be hard pressed to hear the THD in a single tube, FET, or transisitor operating within its design parameters.
Once you have a circuit operating in distortion then you can easily hear differences in how tubes, FETs, or transistors sound when driven hard in stages. People enjoy the sounds of distorted tubes so much so that they emulate them with digital models.
best regards,
mike