before you go into dual batteries:
Being that you have soundstream amps and a capacitor, it appears you may have a clue about quality and such. I always run 2ga welding cable off the alternator and make sure my earth ground system can handle the power. 4ga wires to the amp and 4ga grounds to chassis from the amps.
If you have an upgraded alternator cable already you might want to go with 2 batteries after all.
http://www.pirate4x4.com/tech/billav.../schematic.jpg
i used 2 batteries in one car that I had. I ran the Odyssey PC 680. It is compact and quite powerful.
http://www.odysseybatteries.com/applications/auto.htm
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