RCA cables are directional if they are designed with shielding connected to ground on one end. Usually that would be your source/signal originating end.
Cable break in is real. I have experienced it with interconnects, power cords and speaker cables.
There are multiple reasons why you wouldn’t hear any difference. In example…you are using stock power cords, interconnects or speaker cables made with standard inferior wire and terminated with crappy connectors. Or your system or a given component is not sensitive to cable changes, or your system is not resolving to let you hear it, or you just simply can’t hear it. It’s like some people are able to smell their piss stinks after eating asparagus while others can’t detect it. Doesn’t mean it’s not a real phenomenon if you can’t detect it.
jasonbourne do you like asparagus?