This is not quite correct. Distortion in a tube guitar amp is introduced in the preamp, the phase splitter (for push-pull designs), the power amp, the output t

This is not quite correct. Distortion in a tube guitar amp is introduced in the preamp, the phase splitter (for push-pull designs), the power amp, the output transformer, and the speaker cone. There's also some distortion introduced by amps with tube rectifiers, as high current draw causes B+ (plate/anode) voltage to drop and changes the transfer characteristics of the tubes. This is why there's some disparagement of "master volume" tube amps that attenuate voltage before the power amp to get a distorted sound at lower volumes- they lose the phase splitter and power amp distortion. Whatever the electrical design, when amps aren't cranked, they lose speaker distortion. This is why physically small amps are popular in studios- they can be turned up to their own limits to get distortion without being as loud as stage amps. "Hybrid" amps that use a tube preamp and a solid state power amp certainly have some tube distortion from the preamp, but because of circuit design differences and different tubes used in the power stage, it isn't identical to a full tube amp. Not that it necessarily "needs" to be. Digital modeling amps are getting closer, perhaps even close "enough." Play equipment you like to play and get a sound that you like and the engineering details don't matter.