[Big Daddy]

I like option 1. Building a subwoofer is much more fun than buying one. Take it out of the car, modify it, and use it.

External Amplifiers for Passive Subwoofers:
http://www.parts-express.com/pe/show...number=300-812
http://www.parts-express.com/pe/psho...TOKEN=50414889
http://www.amazon.com/Audiosource-AM...8465481&sr=8-5
http://www.amazon.com/Boston-Acousti...8465645&sr=8-4
http://www.amazon.com/TBI-200-su-Sub...8465645&sr=8-5
http://www.polkaudio.com/homeaudio/p...ifiers/swa500/

Also go to the Upgrading & Building Subwoofers thread and look at the pictures of the subwoofers that I build and the external amplifiers. There are a lot useful links in Post #2.