headlights dimming

[supmet]

I just noticed today that if I have my stereo up, my lights dim. Its not a ton, but enough to notice. I have 1 or 1.5 farad capacitor, 6 or 700 watt amp to a 400 watt sub.(I'll check later to make sure, ya I know, I really know my stereo huh) Stock alternator and battery with 78k. When I first installed it I turned the gain up higher than it is now, the bass higher, and turned it up loud, in the garage with the lights off, and the lights didn't dim at all, everything else in the car was the same, just newer.

I watched the cap with the stereo up, and it never dropped below 12 and never got above 14. Its old, it was in another car before this for ~20k miles.

Oh, my battery died like a month ago because I sat in the car and listened to music for way too long with the car off, but I haven't had trouble starting since.

So, battery, cap, or alternator? I know I can test my battery at auto zone, but is there any test for the alt or the cap? I have a multimeter.