DanAustin

1 Rockford p3 12 in ported box

BostonBruins

Since you say you're not much of an audiophile, the kicker upgrade might be a good option as previously mentioned. They're getting harder and harder to come by, take advantage of the forum offer if you have a screw, I think all the scabs are sold out. I didn't want to mess around too much with the audio stuff as I wanted to do other mods, just like you, but the upgrade was well worth it. Very simple install, sounds great. I didn't have the factory Sony system, so I can't speak for the difference in sound/quality personally, but everyone on here seems to love it.

TuxBlackFX4

Stock sony sub blows. Get the kicker upgrade for a great price. I was given a JL Audio microsub recently. Thing is pretty badass, will get it installed here soon when it warms up a bit.

RES4CUE

If you're looking for a bit more for little money then check out sound ordinance 8-BT from crutchfield.com

It's a powered 8" sealed sub that has a small footprint. You can even fit it behind the seats and take Sony sub out to get more floor space.

You just run power from battery, ground it and use high pass wires to tap into rear speakers for the signal.

I had one for awhile and it was great!