capacitor. I would do this with a stock pulley too, and anything at or above 300 watts. Stereos are just all around bad news for alternators, and capacitors lessen the load by eliminating the sudden super hard current draws of bass. If you do buy one and install it yourself, make sure to read and install it correctly. They hold a HUGE amount of power and if shorted can blow themselves up. As to the pulley difference, I do not believe the pulleys make that much difference in revolutions, most manufacturers do not make their pulleys that much smaller-they just make them lighter.