The thicker ground MAY or may not reduce any noise. If your amp is mounted to the chassis, it is grounded, metal to metal. The ground wire is an additional ground in that case. If you have a short ground run of a foot or so, the additional gauge will not make difference. It depends on how many amps your drawing. The book says to ground both amps to the same point. Also, have you tried turning it down a few notches? The noise could also be comming from your RCA feed lines. One way to tell is to UNPLUG the RCA's at the amplifier and turn up the amp. If the noise is gone, than its comming in from the RCA wires. Try re-routing them or getting better shielded ones. Hope this helps.
thanx man yea it may possibly be emi affecting the rca's