Gauge 4 isn't enough? The guys on the subwoofer forums all said to get 4 gauge kit.
For the capacitor, why is it useless? My lights are still dimming with it but it's a little bit better with it. I thought a 10 or 20 farad capacitor would be the solution. A battery would take a little bit more space in the trunk, and I'm not sure I can plug it in directly to my car's battery without doing some electrical management.