Ok thanks, I see.
This is a very confused thread IMHO. Speaker distortion is
not fundamentally less audible than amplifier distortion. I hope my posts haven't contributed to the impression it might be.
However, what's important in terms of audibility (of
any harmonic distortion) is not particularly the % distortion, but also (1) its frequency, (2) its harmonic order, and (3) its envelope (by which I mean its relationship to SPL).
(Also important is IMD, which is not measured here as you mention.)
There are various reasons why two different kinds of harmonic distortion, although similar in terms of %, might be radically different in terms of audibility. Arguably, there are reasons why most loudspeaker harmonic distortion might tend to fall into the "less audible" category for a given % (loudspeaker distortion tends to be lower-frequency, lower-order, and highly level-dependent). But there is no
fundamental difference between the audibility of loudspeaker distortion vs amplifier distortion.
(Sorry btw if this isn't news to you
@Winkleswizard. I'm posting this again mostly to try to keep the thread on track.)
I wouldn't buy an amp with 1% THD, since this level of THD will definitely be audible with at least some signals, and since there are plenty of cheap amps on the market that perform way better.
Having said that, I agree with the part of your post in bold. If an amp produced 1% distortion below 100Hz and 0.01% distortion above it, that would be very strange, but quite acceptable IMO. Looking at it in practice though, and based on the measurements on this site, if amps tend to distort more anywhere, it's actually in the high frequencies.
2% at what SPL though? The speakers on this site have, until a few days ago, been measured for harmonic distortion at SPLs ranging from 82dB to 100+ dB.
If a speaker measures 2% HD in the midrange at loud levels, but in its intended application is to be listened to at only quiet to moderate levels, it may never reach anywhere near 2% HD.
Also keep in mind that music tends to have a spectrum similar to pink noise, which means that for music playback, the SPLs in the midrange are likely to be significantly lower than in the bass. So, just because a speaker measures 2% midrange HD at say 90 or 100dB, does not mean it will play at such levels in the midrange under normal circumstances.
Another way to put this final point would be to say that the intended application (SPL) is also important in interpreting distortion graphs for speakers.