The sub amp combo is a good combo and pushes that sub as far as it can probably go. The best way to increase that subs spl, which was said before, is to build a strong ported box and move the box around to see where it sounds loudest or best to you. Upgrade the wire because you should but I would not mess with a cap unless you want it for show with the digital display etc. Those amps do not suffer from voltage drop unless it is significant, they have a regulated power supply as long as it is between 11-14.4 or so. Big box with big port(s) is the way to maximize most subs spl. Also use Dynamat or another sound deadner whereever possible if you have it in your trunk.
good luck