The Salk subs aolmrd1241 mentioned are really nice. Jim Salk designed and builds incredible enclosures into which he installs a Rythmik 12" or 15" woofer and servo-feedback plate amp. Go to the Salk site just to see what a really well-braced sub enclosure looks like. He then covers the enclosure with rear wood veneers, just beautiful.
Salk also builds the enclosures to Rythmik’s suggested internal volume, which is greater than those used by Rythmik itself. Rythmik uses a 3 cu.ft. enclosure for it’s sealed 15" model (F15), Salk 4 cu.ft. The added cubic foot affords slightly greater efficiency and output. Brian Ding went with 3 cu.ft to keep the enclosure to a size more affordable to ship. The bracing in the Salk enclosures is absolutely insane, the best I’ve ever seen. Check it out!