Clearly that's what the null tester does....
You've missed the point of your earlier comment about the topology being important, not an individual cable.
I don't want to upset you, but you're not talking about ground loops now, which was your point about grounding.
And hence measurable, l, c and r impacts are all measurable.... usually as a filter effect...the null tester would highlight those differences.
But you said the impacts were measurable...why not by that method?
What is "only a part (and not the most significant) of the system"?
If you are talking about the system under test through a null tester, then there can be anything in the system, cables, PSU, DACs, streamers, etc. Simply record the audio signal, change say the mains cable, record the audio signal again, using the same music, and subtract the first recording from the second, you can then see the difference the changed mains cable made...or any change made.
I think you're missing the idea behind a null test. It can be used to test any system, to highlight differences in the audio when one change is made to that system.
There's nothing to stop one using a mic to record the output of the speakers, and doing the comparison of those two audio streams, one with a mains cable change.
And now you've put a fallible, impressionable, poor audio memory, influencable human in the loop....
