You might have blown some noise suppression caps in the H/U somewhere, this would result in a nasty buzz.
Another thing to try is to stick at thing called a...."ground loop isolator" in between the H/U and the amp, or amp and the sub. All it is a little black box that has 2 RCA plugs on each side, it's got some magical properties that help it elimate "noise". It's probably a series of caps, or something I dont really know, but it's helped me get rid of a buzzing noise to my sub.
Ground loop isolators used to be availble from Dick Smith for about $25, if dickies dont have them try Jaycar or an audio place.