How far does it dip down? As long as your not listening incredable loud the alternator should be able to keep up. I mean it will dip a bit but should come right back up. Ive also heard from several people of having bad experiences with optimas when used for car audio, so you may want to have the battery tested to see if its still ok. An alternator will help with the voltage drop but it may not help with the starting problem. It should still be charging between the bass hits. When you let the car idle with the system low or off what does the volt meter read? Id say if you let the car idle for a bit with the system off or very low and it still wont start its problably the battery.