Is it a joke? e.g. you notch out the fundamental then the distortion tones become pure tones you can hear at whatever level, no longer masked by the fundamental. It's not longer a distortion test, but a simple level test.
Not a joke.
Making the point that, doing a level test at home, I can
only hear a sound 60dB lower than my usual listening level in the absence of any other audible distraction (without turning up the volume obviously, most people seem to seem happy to increase the volume and then be delighted they can hear low levels - duh!).
The proposal that I would be able to hear a sound at
that level at the same time music is playing at my
normal level at the same time is absurd - for
me. I therefore deduce that -60db of distortion must be inaudible to
me.
Test yourself and see if it is different for you.
Just a simple, logical (to me) sequence of engineering deduction.
Based on this it would be a waste of time and effort listening for at what level I could discern distortion starting at lower than -60dB (0.1%) because I can't hear that even without the fundamental.
The most credible (to me) distortion level audibility data is that on speakers (and years ago on record players) and they are so high it is no surprise, electronics are probably audibly transparent.
Noise is another thing.
A level one can hear at the listening position is annoying even without music playing if you leave your kit powered up all the time.
Over my 50 years of dicking with hifi I have had a couple of phono stages which were noisy enough and a Goldmund power amp (ironically the most expensive amp I have ever owned by a big margin) which you could just hear in the background.
Other than that no electronics I have owned had high enough noise levels to hear, but I can't put a number to it.