What determines good distortion?

I'm sure there are some things that could be done to even the Mona Lisa to make it more appealing to some, but that would result in a reproduction that is distorted in some way but for good reason perhaps. Would that be wrong?

given the contents of your post, it would be wrong for you (& probably me as well 'cuz I agree with you that distortion is never even if it sounds good to one's ears) but would be right for many others.
Audio is so subjective & it depends on what the listener's objective is that there is no straight answer to your question. (of course, you already knew that! ;-) ).

Bombay,

I'm with you.

Distortion, whether good or bad, is determined in the head of the listener! So it's the listener who makes the only meaningful determination in the end.

Avoid clipping distortions though always! Oveer the long term at least, I doubt anyone with good hearing would prefer even subtly clipped sound using any amp technology to the alternative.

Distortion is distortion. When you hear or/and feel it, it's bad. Whether it is tube or ss doing it, I have low tolerance.

The odd-ordered harmonics (above the 3rd) are used by the human ear/brain system to ascertain the volume of the sound being heard. This is pretty important to know if you want your system to sound like real music rather than a hifi.

If an amplifier has troubles with this, there will be two results- it will sound louder than it really is, and it will sound brighter than the music really is. All human ears are very sensitive to this!

The ear hears harmonic distortion as tonality. Electronics can have the fault of being overly 'warm' in sound, which is caused by the 2nd, 3rd and 4th harmonics. Quite often tubes get taken to task over this, but that really has more to do with the topology rather than the circuit being tube or transistor. If for example the tube circuit is fully differential in design, there will be no even ordered harmonics and so the circuit will have a more neutral presentation.

Conversely you can give transistor circuits a richer sound by building them single-ended- this will result in more of the lower-ordered harmonics.

Linearity of the devices themselves has a big effect on the sound of the circuit as well. The simple fact of the matter is triode vacuum tubes are the most linear form of amplification known. Additionally, it is easily demonstrated that even the most pedestrian tube amplifier will make less odd ordered harmonics than any transistor amplifier; a sine wave generator and an oscilloscope are all that's needed to demonstrate this.

This is why tubes are still around half a century after being declared obsolete.

"This is why tubes are still around half a century after being declared obsolete. "

Atmasphere, I often wonder what the SOTA is in tubes themselves these days?

Are NOS tubes still the SOTA or has tube production technology advanced at all since the heyday of tubes in general 40+ years ago?

I've noticed prices of new tubes seem to be going up. Is that negatively impacting the value proposition?

"Additionally, it is easily demonstrated that even the most pedestrian tube amplifier will make less odd ordered harmonics than any transistor amplifier; a sine wave generator and an oscilloscope are all that's needed to demonstrate this."

OK, if it is so easy, can you demonstrate that please? WHich amps specifically would you select to do that?

ALso remember demonstrating something does not prove it. Other demonstrations with different amps could have different results.

Even if true, does that mean any tube amp is better than any SS amp? I doubt that!

I like tubes in general as much as the next guy, but just trying to keep things in perspective and avoid people jumping to conclusions based on partial facts.