If I were you I would check you speakers for leaks first before spending any money.  One small leak will make just about any speaker's bass reponse suffer greatly.  I always put slicone on every seam during construction and apply one coat of Helman Sealer to all interior peices.  Unless you are trying to rattle the walls or something the HDTs should be going down pretty low.  If you have some skinny arms that can reach through the port in the back you could probably apply silicone to all the joints (seams) by just removing the speakers and radiators.

This is NOT a recommendation, just rambling thoughts.

Martin J. King offers a simple, cheap design for an 18" H Frame woofer that you might find interesting.

See: http://www.quarter-wave.com/Project08/Project08.html

The woofer costs less than $100.00, and the H frame looks to be a fairly simple build.  Make two, add two subwoofer amps and you can push a lot of air.  (You would't need the Jordan part of his design or the crossover.)

Martin sells software for designing speakers, has an interesting website (http://www.quarter-wave.com/index.html), and an active Yahoo forum.

I had the very good fortune to go to a "Zen Fest" many years ago. Steve had some huge horn subs (Imperial's ?) on the back wall.

All of Steve's speakers sounded great.  Then he would add the big sub's and my jaw would drop.  It's not bass or thump or any of those things I think of as bass.  The sound just became whole. The speakers in front of the subs disappeared.  The air of the room felt different.  The subs improved everything.

I think small drivers can only do so much in a large room.  You have to turn them louder and louder to get mid bass and below matched up with the mids and highs.  But they never match, and you end up turning your speakers up so loud they hurt your ears.  Bass reflex, transmission lines, etc. are all tricks to make a small speaker sound like a big one.  It's hard to make it work at all, and I think near impossible in a large room.

It's like watching a bright TV in a very dark room.  The TV will give you a headache after a while because it's like shining a bright flashlight in your eyes.  You need to put a small amount of light on the back wall to balance the brightness of the TV.  The more even the general balance of light, the less eyestrain.  The same is true of sound.

Good luck!  I look forward to hearing your solution.  Definitely ask Steve for advice.