Has anyone checked into either Transmission Line or Infinite Baffle design subwoofers? As I understand it, they are 'build it yourself' type situations, but are quite easy to do. They also are supposed to give excellent, flat response to EXTREMELY low frequencies, and not necessarily at loud volume (like window smashing rock).
The Transmission Line is essentially a very large box with a serpentine channel in it, like a rats maze, the same diameter as the speaker. Making the "channel' the same length as one quarter of a low C 32' soundwave would be approximately 18 3/4 ft, and therefore capable of resonating at 15Hz with the proper subwoofer driver. Not unlike a mitered pipe.
The Infinite Baffle is only practical for homeowners as it requires household surgery. 1 or 2 subwoofer drivers are actually installed in the ceiling, wall or floor, thus making the attic, adjacent room or wall space, or basement the resonating chamber. The word of caution here is that the sound pressure will be as great in that space as in the room where you want it, (so don't make the other room the nursery!!!)
I like the idea of the Infinite baffle since it takes up ZERO floor space!!! However placement can be tricky.
Just a couple of other things to consider.
~S