I know this sounds elementary, but check your RCA's ! check for tears, and/or pinches in the wire as well as at the back of the head unit and at the Amp input.this can cause a ground to the amp signal causing a nasty "DC Oscillation" simply put, your amp will burp out a nasty signal or "POP" and keep blowing subs. The head unit alone(outputs) can be shorted inside the case and cause this same problem. Start tracing