Ground loop isolators are a nice try but, but it may be a more insidious problem.
Do your RCA cables have a shield ground or "shield drain"? That is a small wire, usually just at one end of the RCA cable (the head unit end) that is connected to a ground (preferably the case of the head unit).
Many, if not most, "High End" RCA cables don't have shield grounds. They advertise how well shielded their cables are, but without the simple addition of a shield ground, any RF signal crossing the cable has nowhere to go but into the cable. With a shield ground, that RF noise is intercepted and grounded out before it reaches the inner conductors.

I find it remarkably stupid that RCA cables, especially those expensive super-duper, multi-orgamisc, zero-noise cables don't have shield grounds. It makes me seriously doubt all their engineering claims.