The only good all-on-juan solution is an Infinity Basslink powered subwoofer:
http://www.infinitysystems.com/car/p...&Region=EUROPE
If you can be stuffed looking, you can get these off eBay from America. I've never heard a bad thing said about them. And you can also get a much bigger fancier version if you like. They are expensive, but cheap if you consider what goes into an amp/sealed enclosure and the actual speaker if you buy a custom system...
Gav.