Short List
Here's an excerpt of some from a short list I'd compiled in another thread about a month or so ago (shipping is approx.):
Rocket ULW-10
Amp: 350w
FR: 26Hz - 150Hz +/- 3dB
Dimensions: 13.00" x 12.25" x 11.75" (HxWxD)
Price: $440 shipped
You can also get this in a black satin finish, but the price jumps $100.
Rocket UFW-10
Amp: 500w
FR: 25Hz - 150Hz +/- 2 dB
Dimensions: 13 7/16" x 13 1/8" x 12 7/8" (HxWxD)
Price: $650 shipped
Dayton Titanic 10" MKIII Kit
Amp: 250w
FR: 30Hz - ?
Dimensions: 14" x 14" x 14-1/4" (HxWxD)
Price: $358 shipped
Cambridge Soundworks P200
Amp: 200w
FR: 25 - 200Hz +/- 3dB
Dimensions: 13 3/4" x 12" x 13 1/2" (HxWxD)
Price: $355 shipped (found on their eBay store)
AV123 x-sub
Amp: 150w
FR: 28Hz - 150Hz +/- 3dB
Dimensions: 17 3/4" x 9" x 12 7/8" (HxWxD)
Price: $240 shipped
Tall at almost 18", but small footprint at 9" x 12 7/8"
The x-sub may be a bit small for your listening area, but I thought I'd throw it in there as well. Other than these, I'd suggest the Velo SPLs as well for this size footprint, though I think you'd have a tough time finding even the 800R for $600 or less.
While the Titanic and CSW subs aren't terribly aesthetically pleasing, they're good subs for the price in a small form factor. -TD