That's a pretty sizable room. Although $400 won't buy you any SVS products, I'd still save my pennies and order a PB10 or PB12. At low volumes, you'll still realize better low end bass with those two as opposed to an Infinity, Cadence, or Velo. $400 is a tough budget, because it's slightly higher than entry level subs, and just under higher end subs.
Had you said $300, I'd say go for something like this:
http://www.ubid.com/actn/opn/getpage.asp?AuctionId=901095984
For the extra $200 or so and SVS's PB10 or 12, you'll see lower Hz if you're really into movies.