I just took a look at the fuse rating for your amp. 80amps x 2. That's 160amps total. If I were you I'd lose one sub and run the remaining with 500watts @ 4ohms. Otherwise I don't see how a big three or cap is really gonna help your situation.
You're pulling too much power. Reset your gains too. You don't need that much power going to your speakers at all times. It can't sound good. Especially since I'm sure you have stock mids and tweets which are over-powered by your kickers.
Just to give you a heads up, lots of power means nothing if it's dirty. People use to win competitions with 25watt amps. Technically, that Hifonics could be rated at 5 watts if you change the testing variables. I bet it only pushes 200watts if you lower the THD requirement to .05%
EDIT: sorry if I sound like an ass. I'm not saying you have a bad amplifier at all. It's actually very good for being a sub amp. But it sounds like you'll end up spending quite a bit of money for "boom" if that's what you want to maintain.
