Upgrading to a 12-inch subwoofer - will it drain my battery?

Don't just assume a bigger sub will kill your battery without checking the actual power draw specs - that's where people go wrong. A 12-inch subwoofer's impact depends on its RMS rating and how hard you're pushing it, not the size alone. A quality powered sub rated at, say, 200-300 watts RMS will drain noticeably more than a smaller one, but modern car alternators handle that fine if your electrical system is in decent shape. If you're already running a lot of other gear or have an older battery, you might need an upgrade there, but the sub itself shouldn't be a dealbreaker.