Naked Buell

Two possible solutions. First, you could add a stiffening capacitor. It'll help if you're just going over on the big bass hits. But if your overall draw is more than the cars electrical system puts out, you'll find your battery going dead even while you're driving down the freeway.

The other fix is to upgrade your alternator. With 600 watts, if there's nothing else drawing big in the system, a 90 or 100 amp alternator should fix you nicely. Don't overlook the wires. You'll want to do a 'big three' upgrade. That's your supply wire from alt to batt, ground from batt to engine, and ground from engine to chassis.

Also, check the amp for heat. As it gets hotter, it will lose efficiency and you'll turn your stereo up higher to get the same output. Same with the woofers, but a ported box won't likely fit very well in your Brat.