IM distortion is incredibly more complex than just that when it comes to audibility and importantly the test protocol-procedure it is applied to ... "The two tones have equal amplitude and they are one octave apart. The signal peaks are about 1.8 volts, and the distortion peaks are about .09 volts, or 5%, and the ratio of rms averaged distortion divided by the rms signal is about 4%. This distortion doesn't look so bad, but it is obviously higher and more complex than singletone distortion." ... this supports my point that measurements are meaningless unless used in their correct context
You are correct that context is very important, but I maintain that specs are even more important. I'll try to break it out logically:
10 percent IMD might be benign on two frequencies an octave apart, such as a solo flute and it's prominent second harmonic. But on a cymbal crash the low-mid tones added by the IMD will be obvious and irritating to hear. IMD on cymbals and tambourines and spoken voice is especially nasty because it adds low frequencies that are not normally present with those sources. So the obvious conclusion when buying a new amp etc is to look at the specs, and disregard anything with unusually high levels of IMD. Yes, it may not be audible on all sources, but it will certainly be audible on
some sources. So it's definitely not a useless spec.
The same goes for THD and frequency response. As Ponk said to Mike a few posts back, "So these amps will make warm, sweet speakers with a bass-extension emulating low midrange hump and a harshness-reducing upper midrange dip sound wonderful, then they'll turn right around, without any eq, or any other changes in the system and make dry, treble-forward speakers with a lean bass sound wonderful too?" A coloration that sounds good on some sources may sound bad on others, and have little affect on yet others.
To me there is more to this than just the 4 parameters you feel explain audio production
As soon as you show what else there is - being excruciatingly specific! - I'll be glad to update my opinion.
it still needs to be shown how those 4 parameters correlate to what we hear
It sounds like you're talking about psychoacoustics, which is unrelated to gear parameters. Further, and I'm sure I've said it a dozen times this past week alone, what you hear one moment may not be what you hear the next. You could easily like the sound of some recording on someone's system today, but hate it tomorrow. This is inside your head and has nothing to do with what happens inside the gear. So it's not proper to bring up "how we hear" into this discussion. I'm talking about gear parameters, not the foibles of human hearing.
if I say "your 4 parameters" often it is not intended as an insult
I never took it as an insult, and I call them my four parameters too. But to all who believe there is more to
assessing the fidelity of audio gear than these four parameters, please tell us specifically what more there is using quantifiable objective terms. "Resolution, transparency, soundstaging" are all useless because they mean different things to different people, and they also have no metric.
--Ethan