With musical instrument applications, you are producing a sound rather than reproducing sound. There is a huge difference. The distortion in a tube guitar amplifier is designed in and is a vital characteristic of the sound. There have been solid state guitar amplifiers made, and they have never supplanted tube units.

Solid state amplifiers typically use large amounts of global negative feedback in order to produce low amounts of distortion and acceptably low output impedance. Tube amplifiers use much less - from zero in the case of an SET, to about 20db in the case of an ultra-linear amplifier.

Additionally, the spectra of the distortion matters as much as the static measurements of level. It is a complex topic that cannot be reduced to a simple statement that distortion measurements are everything.