What Subwoofer is the best bang for the buck?

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.

According to the big subwoofer shootout thread over at AVS AV123's new MFW-15 or the Epik Castle would appear to be better values than several other worthy choices, espcially considering the $600 MFW-15 lands a couple rungs higher than the $5000 Velodyne. Of course one would need to try and listen in his own setup. I have a Selah Audio Whomp.