I subscribe to a photo website, Ken Rockwell, who incidentally reviews a little audio gear. Here are excerpts from his tube amplifier observations.
Parenthetically, I have bridged ABH2s for my Harbeth 40.1s. I do think he makes a good point.
“Tube amplifiers sound better because of the euphonic distortions they add to the music, as well as plenty of other reasons I'll cover below.
Tube amplifiers measure poorly in the lab specifically because of these added distortions, but these distortions are often a part of what make them sound better.
Tube amplifiers have much more distortion than solid-state amplifiers, but most of it is second-order, which is quite musical. That's why it's called "harmonic" distortion.
Second-harmonic distortion is exactly the same note, an octave above. Ditto for higher-order even harmonics; they are also the same note more octaves above.
Even-order harmonic distortion can be so pleasant that back in the 1970s the Aphex Aural Exciter was very popular in recording and broadcast specifically because it was designed to generate and add these harmonic distortions! You can still buy it today”