One thing you can do is check your grounds. Get a cable around 20ft. Attach one end to the neg terminal of the batt (reference ground). Then with a voltmeter, check to see if theres any potential voltages on your (amp and carpc) ground points. Just put one dmm lead to the ground point and the other to your wire.
If you see any voltages or most importantly if the two grounds read differently, then you might have a bad ground.
Did the isolators help at all? Any change?
Maybe try grounding the rca's if the above doesn't help?