I think it is not only the physical aspects of hearing, but training yourself what to listen for. Hearing and listening are two different things. That being said, I have evolved a sense of hearing to appreciate differences in sound with equipment. As far back as 1974 I could hear a BIG difference between a Pioneer receiver vs a Sherwood. And a bit later around 1982, between a Dual 1229 turntable and a SOTA Sapphire. Now I can discern differences between preamps amps and SOME CD players. But once I got into cables, I could not tell much difference once I replaced the cheapo cables that came with lesser components (of course higher end components do not come with cables at all).
For more subtle differences, A-B tests are not very informative. One has to live with a certain component before introducing a change to perceive a difference. Yeah a A-B test will show the differences between a Dual and a SOTA, immediately but not so between a SOTA and a VPI for example. And I would think between high end cables, it would take a lot more concentration and listening skills. I just have not been so inspired to put myself through it at that level.
So, for those who CAN hear a difference, I believe you. For those who can't I believe you too. But, yes, there is some hyperbole and emperor's new clothes syndromes in audio, as there is in just about everything.