Or heck, if you want loud, good air displacement and a 'middle ground' setup, just get a few Kicker CompVX's, 15". 4 subs at 1000 watts should make your balls tingle a tad.
At 260$ shipped, and probably cheaper online in E-bay STORES, you can easily afford your system. Get 4, pay some one to custom fiberglass your trunk so that the subs face angled out towards the back of the trunk. Then, place your 4 Subwoofer amps in pre-made fiberglass shelfs (molded into the middle) so that it looks all pretty and Fueng Shei (sp?)
Heck, you can even get an extremely nice high end SQ set of componenets (Rainbow Germs?) and a sweet high end amplifier (Arc audio maybe?) so that you have the real nice Sound quality in cabin, and an abusive power hitting trunk installed with goodies.
I'd wang it if I had a good 4k to spend. Heck build it myself, would I.
4 subs :1200$
Custom box: 300$ Fiberglassed, and 50 hours for design (I hate math)
Wiring: 100 feet of no-name 8 gauge, 100 feet no-name 1/0 gauge. :150$
3 sets of Rcas, no-name fuses and fuse holders, 1 massive distro block, remote, etc... 100$ at most
2 Yellow tops - 300$ I think
1 massive alternator: 500$
Big 3: See 100 yards 1/0
4 sub Amps: 1200$ to be safe?
1 component Amp: 500$
1 set of components: 400$
200 yards of rammat, appropriate ensolite and glue: 300$
I think I hit it all. That tallies 5k with probably well overestimated prices. I get no-namer stuff at much cheaper than I stated, but I don't know how you are going ahead to buy.