how many pre-outs are coming out of your headunit? sounds like you have a few, try using a different set or maybe try new RCA cables (i know you said they were new). if your amp doesnt have the speaker level input like pudd-dogg mentioned, you can try a line-out converter that taps into your speaker wires. but i wouldnt see why you would even need this if you have preouts in you headunit. but it is an option.