Hugh, you are essentially saying that whether or not changing cables changes the signal/sound is dependent on how robust the hardware is to variability in cable parameters. This is a very important consideration for line level signals and I agree completely. With microphone cables, however, it seems to me that the output circuitry of different microphones is much more variable than that of line level devices, and the signals are orders of magnitude lower, resulting in lower S/N ratios than for line levels. Does your point carry over to microphone cables? In other words, do you think that the sound quality of high quality microphones should not vary audibly with changes in cables from normal studio quality cables to novel innovative cables?
Hugh your point about not testing a cable in isolation is key and shows that the simple test procedure I proposed is probably overly simplified and impractical, at least for microphones. It is easy to imagine significantly different results for different microphones, microphone / preamp combinations, or even different voices.