Mike... you can still wait out and score Phoenix Gold Cyclones on eBay. These provide flat high output down to about 10Hz, no problem, and they are not huge.
Imagine 4 of them added as sub-sub woofers in your system. You can probably fidget an F3 around 6 or 7 Hz out of them with some DCX processing.

Normally, I would consider such pursuits unusual - but in your case - it would surprise me for you NOT to do it.....
-Chris