at any point does the power wire run next to or cross the RCA's? Try to make sure that the two are seperated at all times. Also, if the power wire has to cross the RCAs, make sure they cross at a 90 degree angle. The EMF from the power wire will not enter the RCA cable if they cross at 90 degree angles.

One other thing, what kind of RCA cable are you using? Cheap black ones with red and black RCAs or good triple shielded ones?

Mish