Ahh, car audio. It's been a long time (probably 7 years or so), and with my wife's purchase of a new head unit (and my discovering an MP3 CD my buddy Keith made for me a while ago), I'm remembering what it was to be a basshead.
Just got done perusing the JL Audio website (no other subs will do, really), and figured it would nice to be able to predict the frequency response for some 6W0 drivers in a teeny-tiny enclosure under the rear decklid of my Corolla.
I was duly impressed with the low frequency extension of the stock drivers in the car, to be honest. The stock head unit's frequency response (and power output) left much to be desired in the low frequency range, but the drivers are remarkably capable, and there's decent cabin gain...enough that I grinned when I listened to some old Bass 305 and Tekmaster PEB.
Anyway, the link above goes to an aging (but relevant) page with links to a multitude of software related to audio design (box and port calculators, enclosure design software, etc).
This could be a LOT of fun :)