I built a couple of the 12" 350W servo feedback subwoofers from Rhythmik Audio:
They are awesome, I'm a soundman and have the Smaart Live analyzer and mic that I use to set up large systems. These were easy to tune flat to 16Hz in a pair of 1 cubic foot (real small!!!) airtight boxes made of 1" MDF and stuffed with Dacron. I had the cabinets built by a guy Brad @ Woodstyle products 626-968-2830 that did it for reasonable money and finished piano black. They look about like the ones in the first picture on this page:
This was the first time I ever really saw 16 HZ low end response so easy to get out of a 1 cubic foot box!. Using the extended setting on the amp, there wasn't a lot of external EQing to boost it or anything. They also have a 15" version, but my room was small (14x18) and space was at a premium.
These really are concussive on explosions and even just good car door closings. And the musical SQ is very transparent. Once you feel the bottom octave that's usually mostly missing or over-compensated for in the next octave, you'll never want to go back.
I used a Berringer Feedback Destroyer Pro for it's parametric EQing ability so between that taking out the standing wave room modes, and the servo feedback amplifier controlling the cone and system response, it's just right.Edited by genesound - 10/31/12 at 9:01pm