Subwoofer reccomendations for B&W 805S?

My room is 17x15. How big of a sub should I need? Does anyone have any good reccomendations?

I pulled the specs for the 805 and I don't believe that B&W quotes a crossover spec for these speakers.

The active device between pre and power amp is an active crossover (like the NHT x-2 I use) that allows you to divert the low frequency info from your main amp (and the 805s) to your sub(s).

As I noted in my post, if you move your crossover frequency much above 100hz, you should use 2 subs run in stereo.

As I said earlier in this thread: The Rel Strata III is a great sub with the 805's Great deals can be had on Agon.
I heard the Paradigm Sub One. I was amazed at the clarity and slam. I liked it a heck of a lot better than the REL. I think it is more expensive however, but both are in the rarified air.