If it were me... I'd look into something like the
JL Audio cleansweep or the
Alpine version. JBL supposedly has one too with logic 7 and modest amplification, but I can't find it on their site. Good 6x9's with a healthy amp and a trunk as an enclosure can produce some respectable bass, you might not even need a sub.
I'll offer some unsolicited advice on speakers. The big boys run a pair of front speakers and sub(s), no rears at all- they call it "front stage". Buy 2 great speakers instead of 4 average ones. Try fading your deck all the way to the front for a couple days and see how it sounds. I was skeptical at first but now I'm a believer. The trick is to power them well- HU power won't cut it with only 2 peakers.
