The mini-dsp is cool, but I would go with a DCX 2496:
Mono signal or up to 3 channels > DCX > amps > up to 6 subs.
It's not diy, but the DCX is versatile and cost effective at a street price of ~$260.
It has balanced inputs and outputs, and separate XO, level, phase, delay and 9 PEQ for each channel. The sound quality for subs is excellent without any mods.
Doug