Test light + Car Audio = Junk.

Go down to Sears and grab a 20-30 dollar auto range Digital Multi Meter and use that in diagnosing. Pull the RCAs out of the amp and turn your meter to AC current. Take the neg lead from the meter to the outer shield of the RCA, and the positive lead to the prong sticking out from the middle of the RCA. Now with the music low, turn up the volume and you should see the AC current rise. If it has no effect then your problem is either in the cables, head unit, or hi-low converter.

If RCAs are good then take your meter and test the output of the amp....again neg. to neg, and pos. to pos. Again volume low, then turn it up to see if the AC voltage rises.

Next test your Subs....turn the meter to the Ohms setting and test.