That's a 4000 cu. ft. room, quite large. Your best bet is to get as good of a sub as you can right now then save up for an identical second one. You didn't specifically mention a $ figure, but I would stay away from 8 inch woofers, they just won't have the oomph in that big of a room, even with 2 of them.
There's a budget subwoofer thread by Jim Wilson you might want to look through. Personally I'm looking at a 2nd SVS PC-12NSD, they have some PB-12NSD which is sonically identical, just a box vs. cylinder, in their outlet for $629, which is a killer price for that kind of performance. It is ported, and generally speaking for music it is said that sealed is better (I'm not so sure about that, a good sub is a good sub, people, please don't flame me!) and I am very happy my my ported sub. The advantage of the ported is more output, which you will sorely need for that large of a room.
Look around and let us know what you find that interests you.