What Subwoofer is the best bang for the buck?

Parts Express, any model, especially the 10" at $125

JL F113 and SVS Sounds PB13 Ultra are the best value in output and reasonably low distortion. You can do better than these but nothing else beats them in terms of "bang for buck"...IMHO.

I think the Velodyne DD series is phenomenal, and the prices have really come down.

Hsu research. They always have the best for less, in all sizes.

You should check out MJ Acoustics Subwoofers. They are very similar to REL, the company was founded by former REL employees, and work very well with full range speakers. They retail around 35% less than the comprable REL Subwoofer and are even less expensive in the US where they do not sell as well as they do in the UK. I have owned them all and my MJ Acoustics Reference I MK III is very very good. It is as musical as my former REL Stadium III for half the price.