in reality, you have the wrong car for your car audio application. hybrids like that are for people who DON'T want to put a massive sub stage in a car. but, you can make it work. you'll just have to behave and ensure the 12V batteries are being charge properly by monitoring their resting voltage, some days you'll need to take it easy and let them recharge (or plug them in at night). if you don't drop below 14.4V at idle, you know you aren't drawing much power at all. what is the 12V output of your alt at idle and at 2500RPM? i'd guess around 60A at idle and 110A at 2500. That means you can deliver about 500W RMS to a sub at idle while maintaining 14V, and double that at higher rev's.
if you aren't satisfied with your existing sub and amp combo, it's not that you need more power, it's that you need to properly integrate that sub into the vehicle. it means you have cancellation and losses through the thin, flimsy trunk walls. more power in the same space won't make you as happy as just doing a proper install. that's not a dig against you, most people just toss a box in a trunk and think that's all they need. getting the vehicle ready for something it wasn't designed for is the biggest challenge and usually overlooked.
