First off, with your budget I recommend either Dayton Reference series subwoofers or Acoustic elegance subs. AE are a lot more expensive but can play really high frequency for a subwoofer.
I strongly advise you against the Titanic MK subwoofers if you are listening to music. I have used them in car audio and they are really boomy. They can't be beat in total output, but I feel the reference series are much more dynamic, and all around better sounding subwoofers. Go to diyma.com/forums
; they did a kippel test of the Dayton reference 10" HO subwoofer and the performance is stellar.
Second, I can't see why you would want (2) subs in a room that small. Well, I can see why you want two, but I can assure you (1) sub properly sized to a custom box will make the fillings in your teeth hurt. Especially if its (2) feet behind your kidneys (behind couch?)
So for $150 delivered, I recommend this
. Its the Dayton 15" Reference subwoofer. Since you have oodles of room behind your couch, build a 5 cu.ft. box tuned to 15-20hz and get either an O-Audio 500w plate amp or a Dayton 500w plate amp. I can model this for you if you like when I get home; I just took the recommended box size from the Parts Express recommended box thread.
So with the Dayton 15" HF ($150), a 500w plate amp ($220 approx), and a custom box ($32 MDF (cut to size at Home Depot), wood glue ($5), screws ($10, a 4"-5" diameter (port tubing either at home depot or pro-port from parts express) $20, you can have a world class bass experience for:
($150 + $220 + $32 + $5 + $10 + $20) = $437 approx.
Thats what I would do. Hope that helps. Your neighbors will despise you.