@blackgtp-----what are you going for crazy lows that make your teeth shake or just low enough to be tolerable and have plenty of sound quality.
FWIW the last 4 subs i have put in my different cars have been Infinity Reference 12's. i have run them 1 sealed, 2 sealed, 1 ported, and 2 ported and i have to say that they have performed great in every application, just enough bump but without sacrificing sound quality, and they can be had for a pretty cheap.
Also make sure you get the right head unit. Pioneer does wonders for subs\bass because most of them have HIGHLY customizable EQs rather than 3 or 4 presets. If your HU doesnt have a tweakable EQ then consider getting at least a 5 or 7 band external EQ and mount it either under your HU or somewhere inconspicous. Also your headunit should have a sub control in it, you don't want to have to run a remote knob for your amp if you don't need to.
@chamilton89 --- i totally agree on the amp comments. even at 300w you can get a sub to hit pretty well. also keep in mind the more power you are trying to dump into your subs the more likely you are to need a capacitor, and the more power you are trying to push the bigger cap you will need.
