Well that would be interesting. What that Mr. Morello is complaining about in the thread I referred to is the measurement itself. I'll translate a little more. He got counter questions. Other than that, I only provide what is written. Do not shoot the messenger. For those who are interested, you just have to go into that thread. Enter google translate.
Morello wrote:
I do not think you understood what I wrote. If a power amplifier exhibits THD of 1% at 20 kHz with a dominant third tone, it means that the third tone has a frequency of 60 kHz. If you measure with 25 kHz bandwidth, the third tone will be attenuated by, for example, 40 dB (depending on the flank attenuation of the filter) and the measurement will show THD of 0.01%. The measurement thus gives a false result.
Belker
I understand your argument, and Amir and co certainly do, but they do not seem to think it is a practical problem. Or they think otherwise. In any case, they do not seem to think they are making irrelevant measurements.
Morello
Again: the question is not subjective and something to think about. If THD is A% and the measurement shows B% and where A is significant (beyond reasonable tolerances) separate from B means that the measurement is irrelevant.
I-or
What Morello has tried to convey above (and in many older threads) is that the nonlinearities that give rise to inaudible spectral components above 20 kHz when measuring THD also give rise to clearly audible intermodulation distortion (in the audible range). If you measure IMD instead, this will be clear.
......And so on in that thread ...