as said in another post, to each his own, car audio is personal preferance on about 95% of stuff. Depending on the quality of your subs you'll get dfifferent results, sure you can buy 3 12" subs and they'll hit and move some air, but they may not sound good while doing it, it could just sound like ambient noise. What i've learned is the better quality woofer you buy and the better quality box you build, the better it should be...i use should because sometimes subs do just suck
You can have subs that have both good SQ and good SPL, but there's a trade-off somewhere. You can go buy stuff that'll sound shitty and destroy your hearing in a heartbeat like an RE MT or something, something in between like an X.X.X or Eclipse Tis or a whole slew of other SQL subs, or go very SQ with something like an IDMAX. Your best bet is to go to a shop and hear some subs.
in my experience, the box, crossover points, and EQ affect the quality and charactor of the bass far more than the actual model of sub you buy. however, the design of the sub itself will determine the level of bass performance you will get out of a particular system.
for example, if you take a $100 sub and a $600 sub and put them both in separate enclosures that make similar responses, they will both sound very much alike up to a certain output level. above this point, the $100 sub will give out and start to sound quite bad, while the $600 sub will continue to play louder with good SQ and SPL.
if you are looking for lots of SPL, you will need to use better-than-average subs (big ones if you can), and lots of 'em. you will also need the power to drive them. it adds up to big $$$ real fast.
You hit the nail on the head. The equipment you use accounts for about 10% of how your system sounds. The other 90% comes from the installation, be it for component systems, amplifiers, head units, or subwoofers. The best equipment in the world can still sound like crap in a bad install.