Well, as to distortion at lower levels, the usual 90dB NRC graph is also shown and the distortion is low enough that at even lower dB levels it can be assumed that it wouldn't be audibly significant. Of course, note in the NRC measurements description that they point out that "90dB measured anechoically is very loud and considered far beyond normal listening levels..."

Incidentally, the formula for calculating the dB down distortion level is 20log(distortion percentage/100).

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Enjoy the music, not the equipment.