Trigged by this thread, I decided to do an a/b comparison between a cheap RCA cable and a relatively cheap optical cable, connecting my CD/DVD player (Harman Kardon DVD 22) with my receiver (Harman Kardon AVR 130). I have cheap but rather thick speaker cables and a pair of average quality Jamo speakers that I, TBH, have never been very fond of. There was a slight difference between the two. Much smaller than I believed it to be, but a difference anyway, to the favor of the optical cable. Maybe it would have been bigger with a better pair of speakers. I think you need to have really high-end equipment to have any concrete use of more expensive cables. But $500 cables?