I second the Rythmic sub but would steer you towards the GR Research version. GR actually offers a kit of this where you can buy just the amp and the driver and build the enclosure yourself. I spent $500 plus shipping on those 2 pieces and another $60 for mdf and got a killer sub out of it!
Absolutely phenominal performance with 20Hz extension with a -3dB in the teens. It has added depth to music in a way that I've never heard from previous subs I've owned. (B&W, Definitive)
GR has free plans available for a well-braced cabinet or you can DIY. They only require between 1.5-1.8 cu/ft of space per driver. And the amps can drive up to 3 woofers.
Here's mine.
