[Arran]

I'm not sure what they used as a signal source to measure the distortion levels either, I assume an audio function generator with a series of fixed tones, but there would surely be some variation between generators, and one tube amp, or solid state amp and the next. One thing is certain, there is almost infinite variation between piece of music, and the quality of the recordings, if you tried playing an Edison cylinder record through even the best audio amp ever made it would still sound terrible
Regards
Arran