"If someone can't remember what something sounded like a few minutes ago they're not listening very carefully"
You obviously don't understand the issues with human memory capabilities. Not that I do, but I know memory is hugely fallible.
"That requires a switcher box. If we assume for the moment that speaker cable has an influence on sound, then surely a switcher box would too."
yep, it's a problem, but a really, really good switcher maybe could help remove that argument to a large extent?
I think most cable differences are probably just in the very high frequencies. I don't hear specific tones over 14khz, but I still seem to sense higher frequencies (I'd kind of like to DBT that, since its from self testing). Perhaps our brains use higher frequencies in some way for spatial sensing or something. Other animals seem to have these capabilities, and who knows how conscious they are of audible sound vs other brain uses for sound? It is not silly to think that some animals perceive higher frequencies different than they do lower frequencies. Animal brains are quite strange. Bats use of "hearing" is extremely different than ours, for example.