Dr. Geddes has said before a few times that lack of crossover distortion is one of the key elements he looks for in an amp assuming an otherwise competent design. It so happens that the Pioneer receiver is one of the few budget amps that meets this criteria. Class A amps (either SS or tube) are inherently crossover distortion free.
He has also said that much of this isn't audible IIRC. I got the impression that he didn't choose the Pioneer so much because it sounded better, but rather because it measured better.
CD is hard to measure but it is audible. Most SS amps are Class A/B and pay little attention to crossover distortion.
A while back I remember seeing mention of a listening test where the amp that people preferred over the others was Class B. It made me wonder how audible crossover distortion actually is.
I think there is also the spectrum of the harmonic distortion that is largely ignored but more important IMO than the level of HD distortion assuming we are not talking about gross (>3%) distortion. I'd take an amp with say 1% 2nd Harmonic distortion and immeasurable upper harmonics over an amp with neglible 2nd HD but trace amounts of 5th HD.
What you say is true, but at what point does it actually become audible? I think we once discussed a Parasound amp that had upper harmonics. You said you wouldn't want it for that reason, while I wondered if distortion that's 100 dB down or more is audible.
THD is a useless measure because it doesn't take into account this feature of how we hear.
It is probably useless for loudspeakers, but it can still give a reasonable indication of when an amp performs well. I'd say that if an amp (that Parasound A 21) has less than 0.03% THD broadband at normal listening levels, we're good to go.
On the other hand, if we're looking at a SET amp with several orders of magnitude higher THD, it is worth looking at the distortion spectrum.
Some amps at, say, 3% THD will be truly noxious, while others may be more benign, or even in some cases pleasurable. (Euphonic distortion.)
