Have you ever thought of building your own subwoofers?
I started out with an SVS tube, then built my own sono tube sub and now have built two 18" sealed monsters (size wise). You can see the build in this thread.
If your looking in the $1,400 price range each, you could base your build on the TC Sounds LMS 18". One LMS 18" in a box with 2 TC Sounds passive radiators driven by 2,000+ watts should be able to shake the foundation of your house. This setup would also be idea for your layout. The optimal placement for two subs is front and back of the room but your layout doesn't really lend itself to a rear sub.
The other option I would look into is a Danley DTS10 or DTS20, some
feel it is the standard all other subwoofers should be compared to. They even sell the DTS10 in kit form. You have pleanty of room behind the screen for a sub of this size and if you want a lot of output below 20Hz, these may be the ticket.