Here are some subs that I would wholly recommend:
http://www.partsexpress.com/pe/pshow...number=299-670 - Great buy for a Soundstream Sub.
http://www.partsexpress.com/pe/showd...number=295-185 - Even though this is a no-name brand, it is a better sub than your typical Rockford-Fosgate, MTX, etc. subs than you find at your local BestBuy, Circuit City, Cruthfield, etc. Do an internet search for Dayton Audio subs and you will be amazed at the number of people using these subs for DIY (do-it-yourself) projects mainly for home theater...which is much more demanding than car audio. Look at the frequency response (look for subs that go as low as 20Hz - lower is better) and Xmax (distance cone will travel - bigger is better).
Here is one of Dayton's best subs:
http://www.partsexpress.com/pe/showd...number=295-414 - This is one bad *** sub at a wonderful price. Used extensively in home audio by true audiophiles. Name brand subs DO NOT compare to this sub at this price...not even close.
I have to agree with others here that ID and Adire are awesome subs as well. My future project is to build a new home theater/audio sub using the Adire Audio Tempest (15"). I already have the driver and it is a beast and a tremendous value at $125. Tempests will go extremely low and they have excellent sound quality.
Check out some Adire installs here:
http://www.adireaudio.com/StockProducts/installs.htm