I am skeptical of "evidence" gathered in the 1920's and 1930's about the audibility of the harmonic spectrum and distortion spectrum of amplifiers and how it might apply to present day audio electronics. Without knowing more I tend to assume (with all that implies) that much of this is empiric data that has not been subjected to appropriate testing to establish it as true evidence. That is not to say that it isn't useful in amplifier design, where the designer will have both measurements and listening tests to attain his design goals, but how it relates to the consumer's (listener's) ability to choose the "best" amplifier for him is unclear.
Your skepticism does not serve you well in this case.
If you had looked up the reference, you'd have found a page that referred to the orders of harmonics. This one has tripped me up in the past so I'm going to elaborate on the issue so hopefully you won't be tripped up in the same way.
The 3rd harmonic is 3X the fundamental tone. But musically its a 5th. That is why it can be innocuous if not too loud. But the 7th harmonic, 7X the fundamental, is not a harmonious chord. IOW harmonics, with the exception of the 2nd, are not octaves above the fundamental tone. That is one of the reasons the 7th has to be very low or else it will grate on the ear.
This simple fact has been understood for several centuries.
Now something else is pointed at in that 1930s tome; the simple fact that humans are far more sensitive to higher ordered harmonics than they are the lower orders. This has been repeatedly proven in the succeeding decades, but unfortunately largely ignored by the audio industry for most of the last 70 years. The reason we are so sensitive to these harmonics is the ear uses them to tell how loud sounds are, and since the ear has about a 130dB range, that means it has to
really sensitive in this manner!
What I'm talking about might be an amp that is low in distortion, perhaps only 0.01%. Such an amp might measure quite well, but when you listen to it, its harsh and bright. That's because the higher ordered harmonics compose most of the distortion spectra- the lower orders for whatever reason are less prominent or more greatly suppressed. Yet this fact could have been surmised by any designer having read the Radiotron Designer's Handbook from that long ago.
Knowledge is a cumulative thing until something comes along that explains the phenomena better. That's the nature of what we call 'theory'. The simple fact is nothing has supplanted the theory documented in that 1930s text.
Enough about that. The thing now that should be easy to see is that low THD simply isn't enough to tell you anything about how an amp sounds. Further, if you know the distortion spectra then you can take a pretty good guess at what it will sound like. But most audiophiles that promote the idea that we 'can hear things that can't be measured' never got to that bit about distortion spectra. They are still hung up on the THD. So we have this myth out there which was once true; nowadays though we can measure distortion spectra fairly easily and we can measure distortion vs frequency. If we understand what all this can tell us, then the idea that measurements won't tell us how it sounds simply winds up being false.
But like I said, those measurements rarely get published if they are even made. A lot of equipment would be shown up in a big hurry and I have this idea that the industry for the most part really doesn't want that. They'd have to fixs all the bugs in their designs- and that is a lot easier said than done!
