Jasper said:
it'll thump to the end of the earth, and the SQ is absolutely unreal.
Cmon now, lets not get too out of controll here. Its a killer setup for the money....yes. However I'd put even my O series 12 up against a VQ anyday and bet it would not only out SPL it, but out SQ it as well.....
Anyhow, IF you were to replace your factory sub you would have to get an amp to power it. IF you were to do that, you would want to get another sub to justify the amp.
May I suggest:
IF you just want a little more bass, and want to keep your trunk space.....
Get some 7 inch Vifa's for the rear deck. I have been wanting to try these out for a long time and see how I like it without the BIG bass of two 12's. They are relatively cheap at like 60 bucks each. Then get a nice set of components for the front. Pair those with a nice 4 ch amp and a decent HU and youll be way more than happy. IF you want lots of bass though, do like Jasper said. Amp, Sub, Box, and be done with it.