I agree that a transistor without any negative feedback produces such severe distortion that its sinewave output level is difficult to measure.
Also, a transistor with an output signal level/supply voltage that is at a much higher percentage than a vacuum tube produces a much higher distortion than the vacuum tube.
A very expensive output transformer operating far from saturation was used 60 years ago but is rare today.

But vacuum tube guitar amplifiers were made to produce distortion and even use "overdrive" and "fuzz" pedals to make as much distortion that was possible. Vacuum tube PA amplifiers were made cheap with no regard for reducing distortion.

58 years ago my Heathkit vacuum tubes amplifier was inexpensive and sounded great. Until I took it to a MacIntosh Amplifier Clinic where they measured its 20% distortion and compared it to a MacIntosh much more expensive amplifier producing "only" 2% distortion. I replaced my 6 months old vacuum tubes and it was measured again. Then 3% distortion.

 

Since modern, well-built solid-state amps have harmonic and intermodulation distortion well below the audible point, there seem to be only two valid reasons to use tube audio amps:

1. You think they are cool.
2. You like/want the sound of the distortion they generate.