... Passive sub unit. This isn't too far out of reality. There was something in the test lab at Axiom that looked like the bottom woofer and port of an M80, partially seen at the bottom left corner of this pic.
I was actually thinking more along the lines of 2 woofers side be side and 2 vortex ports, and a cross over for 60-80 Hz and below - maybe on a footprint of an EP800 (trapezoid), but obviously much shorter. The point being to add just a little bottom end to something like M2's without going to a powered sub.
I tried this when I got my T2's and used the bass module from my old Cambridge Soundworks "sub" / sat system and it really warmed things up. It didn't pass WAF at home, but I'm thinking about this for my office at work. I don't want to boom boom the neighboring offices.