Check your wiring to make sure non of the wires are touching ground. If one or more of your speakers are blown, it could cause an open or short (or throw the impedence of the speaker(s) way out of specs). One way to tell if your speaker's voice coil is blown is to physically move the cone in and out from as close to the edges of the dust cover of the speaker coil as possible (not on the dust cover) keeping equal pressure on all sides. If you hear a gritty sound, it could mean your speaker's voice coil is history. On most high-powered amps, it's important that you're not using a common ground (some older inexpensive car units did this so you'd only have to run a single wire to each speaker and use the car body as the ground point for each speaker). I doubt seriously your system does this, but that's why it's important to make certain that the speaker wires are not grounded. You can disconnect the speaker wires from your amp and check each wire for continuity to ground with an ohmmeter.

Also, check the ground point of your amp...if it powers up, the source voltage is probably okay, but there's nothing wrong with running a cable directly from the battery just to make sure.

Good luck...