You would probably be better off upgrading the alternator. Start with the big 3. Your battery is a storage unit, and your alternator recharges the storage unit. Go and play it loud for an hour and see how your electrical holds up. Turn on your lights, your AC, use your turn signals, plug some stuff into your cigaretter or usb, and see if you have any dimming lights or voltage problems. If you do (after the big 3), you need a battery that can put out enough amperage. If you have problems right away then you need a larger battery to keep up with your demand in the moment.
If after an hour your voltage starts to steadily drop, your alternator can't recharge enough to keep up with the demand, and you would need an alternator that can produce more to keep up with what you are using.
What am I missing here: That amp puts out rated 1920w RMS (and aren't those underrated)? That speaker is rated 900w RMS. Unless you are dialing down that amp, its going to kill that sub, if you didn't already.
What year gmc 1500? 05+ or earlier?
Clipping is when the amp is pushing the signal too loud. It is when the wave, or signal is being cut short / clipped. The cause of It has nothing to do with the speakers. I would guess that the clipping and your power issue are two separate things. The clipping would be because of an improperly set gain where the signal is set wrong. I swear I looked at those taramps, and the signal voltage is 3v max. (I could be completely wrong though). I remember there was some setting that was really odd on those.
Your crossovers / pass filters might be set wrong and cutting off what you want and that would be losing your power. If you have a multimeter you can check your dc voltage and see what wattage is putting out.
