Metku wrote a tutorial on how to design and build a high-end subwoofer.

I have a pair of Genelec 8030 speakers. They are good but bass reaches only 55 hz. Music sounds great but movies lack a bit of depth. This subwoofer project is based on 10" subwoofer driver made by Norwegian Seas and Hypex subwoofer amplifier.

Modding Tutorial - How to design and build a high-end subwoofer