Ethan, if a note's timbre changes, then the spectrum of FR will change, that's easy to understand; however, if a given note's fundamental and range of harmonics stay the same, maintaining the same basic timbre, but the individual harmonics are easier to distinguish, then how will that change FR??? I'm going to need some sort of very refined spectrum analysis to see a difference, I suspect.
This is a great question.
I think you are right, that the measurement will need to be refined. I think this is well within our ability to measure however.
I, too, have heard cables with greater clarity - as well as cables that sound dreadful. I am with Ethan in believing that these differences should be measurable.
Ethan's suggestion that we compare waveforms of the exact same piece of music with just a difference of cables is excellent. This is as detailed as we can get as the waveform tracks both frequency and time. If the changes are in teh timing we will see a difference. If they are in frequency we will see a difference.
We do need to record electronic output and not speakers however, comb filtering and all.