If you're open to spending more than you had initially planned, and are ok with that, then DEFINITELY DEFINITELY spend more money!
I've spent the better part of the last few years dealing with a HUGE room like yours, it's about 10,000 cubic feet, and have just come to the realization over time and alot of sub purchases, that for a room that big, if you really want to feel the bass, you'll need a couple of really good powerful subs. I've had as many as 4 MFW-15's in my room, and that did a good enough job. I would have liked more, but they were adequate enough for me.
I've had quite a few other subs over those years, but those MFW's provided much more output than the other subs I've had. I've had a bunch of different SVS subs in there, and the MFW's clearly gave me what I need.
So I would definitely go with duals of something if I were you. CHT would keep you within your budget as they have a pair of 18" sealed duals for around $1250 shipped that you could get. Ported could work even better for you due to the room being so big. If you can wait a few more weeks, than that could be worth it because CHT will soon be offering new vented subs. They will run around 2 grand or so shipped. 18 inch drivers for those as well.
That's going to be my next purchase. Hopefully they will take me to the next level and surpass (by a large margin I'm hoping
) what my MFW's have given me. They definitely should.