If I'm not mistaken, yours is a 150A alternator already. Be glad you don't have a DSM because they come with either a 65A or a 75A depending on whether or not you have an automatic or a Canadian car. Those cars are powered-everything, and many people are running some big systems on them.

What you need is to save your money, buy an OE alternator, and get a 1 Farad Capacitor for your system. That way the bass doesn't drain your battery and it doesn't put a load on your alternator. Your OE alternator would work fine with a cap.