Someone will post with way more knowledge than me but in layman terms, it goes like this:
Your stereo sends an electrical signal via wires to a processor or "Amplifier" and from there the signal is converted and "boosted" and sent to the speakers for sound and the subwoofer for bass.
Basically all a subwoofer is, is a giant "Air mover" and that is what creates the concussive force that you interpret as bass or low level sound.
Here is a pretty good read if you're interested:
http://www.edmunds.com/car-technolog...o-systems.html
I, personally, am looking for a clean sound at a medium to loud volume with a little bit of clean bass.
I would look into replacing the factory door speakers (With a 6.5") and rear deck speakers (Which are 6"x9") with some decent quality speakers.
If you want a little bit of bass, go with a 10" sub and a good Amplifier. If you're wanting more bass, go with a 12" or (2) 10" subs or (2) 12" subs etc etc
MY plan for reference?
Front - 6.5" Polk speakers
Rear - 6x9 Polk speakers
Subwoofer - Single 10" Polk,or something similar
Amplifiers - A Mono (1) Channel amp for the subwoofer and a (4) channel amp for the other speakers.
(1) channel on the amp will power each speaker for a total of (4) speakers (2 front and 2 rear) for all intents and purposes.