Wayne,
Thanks for the tip. Now I've gotten the deep dive on flanking subs and have a better idea of what I'm doing. With the idea of blending in that range I can understand the 12dB instead of 24dB taper.
My understanding is the subwoofer Amp isn't super critical. So something like a Dayton plate amp is probably ok. Looks like the DSP is preferred as hopefully it'll allow for crossover slope control as well as frequency. Do you have any suggestions on power? I don't know if I need to match/go over the 400W rating on the 3pi, or if 250W is fine. I'd be running flanking 3pi subs on a pair of 8pi horns powered by KT120 PP amps (oddwatt style) running 30-50W.