For me, the hardware will be pretty easy. I will be designing it this week while I'm out of town. It will be very small and hopefully of good quality (there is an unavoidable tradeoff between size and quality). The problem for me is the software aspect. I have no programming experience and minimal exposure to C and Linux (esp. Linux), so developing the software would be a very daunting task. Shonky (Christian) has made and tested a breadboarded sub out hack for his Empeg, complete with software, however, I don't believe his software is of the parametric variety you want though. It has preset filter points only.

The sub out board would tap into the "IIS" header located under the 2nd drive bay.

Stu