Well said USG. The mere fact that the differences between headphones, dacs and amps can be easily measured with test equipment, and the differences between cables cannot, proves the validity of your order of magnitude argument. As a general rule, it seems safe to say that measurable differences will have a greater impact on sound quality than system changes which are not of a sufficient magnitude to be measured. If a headphone cable really is capable of increasing brightness or warmth, this ought to show up in the frequency response curve. I have yet to see a plausible explanation for why it does not. IMO - this fact should be stickied at the top of the headphone forum, rather than being generally confined to sound science.
My experience with an aftermarket cable for the 650 was that it did not brighten the sound but rather increased resolution, especially in the treble, which gave the impression of brightening the sound. Could this be why no FR differences can be measured? Perhaps we should be measuring square waves or something similar for evidence of better transient response. In any case I did hear a difference, and each tme I tried to go back to the stock cable, which was more convenient, I ended up back with the aftermarket cable. Why? Just vague dissatisfaction, which I could neither define nor measure.