^^ Some of this is true. Most digital engineers are still struggling with in-harmonic distortions, those that arise that are related to the scan frequency rather than the fundamental.
SET amplifier designers understand that the ear cares very little about lower ordered harmonics, in terms of objectionability. This is true but the ear does hear all harmonic distortion as tonality- this is why SETs sound 'rich'. Its also why they sound so dynamic for their low power.
Transistor amp designers have to be quite focused on getting low distortion, as transistors often tend to have poor linearity. So many of them use negative feedback to acheive linearity. There is a problem with this- the ear uses odd ordered harmonics to detirmine how loud a sound is. When you add negative feedback to an amplifer, which lowers THD, it can add to the odd orders. Those pesky 5th, 7th and 9th harmonics are the troublemakers. Since our ears use them to detirmine sound pressure, our ears are more sensitive to them than the best test equipment. This is why transistor amps can sound bright and harsh, even though they have flat frequency response. Its not because there is a frequency response abberation, its because of odd ordered harmonics.
This is why tubes failed to go obsolete 50 years ago- they sound more pleasent because they make less odd ordered harmonics. But if you want tubes to sound neutral, you have to get rid of the distortion they *do* make which is mostly the 2nd 3rd and 4th. The only way I have found to do this is to build the tube circuits fully differentially, which cancels out all the even orders throughout the entire circuit. And don't use feedback.
The problem with no feedback of course is that its very hard to get low 'output impedance', especially if you want the amp to behave like a voltage source. But behaving like a voltage source does not guarantee that it will sound like music, so we opted for a different approach, which is that if the speaker demands that of the amp, the chances are very high that it will thus never be able to sound like real music.
Overall audio engineering is not as well thought out, cut and dried as we are often lead to believe, simply because the human hearing rules that are emerging due to ongoing research have yet to be exploited. But if past history is any indication, it won't be happening anytime soon. We have known how distortion affects tonality and that the ear uses odd orders to detect volume levels since the 1960s, but as an industry have not done anything about that since then other than argue. So don't hold your breath!