0.1% translates to a SINAD of 60 dB so yes, that is definitely not a useful target to me.
For absolute audibility thresholds, yes. However, music masks distortion, which allows there to be distortion above the room’s noise floor that isn’t audible.
https://www.bksv.com/media/doc/BO0385.pdf#page4
As we can see in Fig. 6, for 100dB [EDIT: at 1kHz], the 2nd harmonic needs to be above -30dBFS to be audible, -40dB for the 3rd harmonic, -60 dBFS for the 8th harmonic, etc.
Thus, simply stating THD and not showing the spectrum of harmonics across the frequency range is not the best to do either, as the same % of THD can become more audible if the distortion is mostly higher order. And of course higher THD at 40Hz can be tolerated in relation to 4kHz, which is why seeing THD vs frequency graphs help as well.
So yeah...it’s complicated. However, I still feel 0.1% THD is a good target. Stereophile uses 1% as clipping usually as that’s what is normally stated for audibility thresholds for music, and the specs for most all the solid state amps they measure list their output at 1% THD.
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