B&W Nautilus Subwoofer to match N804s


I am slowly but surely building my complete Nautilus home theater system. Yes, I haven't eaten anything for a while, but it is worth it. I have purchased the N804s for left/right and the HTM-1 for center. Eventually I will buy two more N804s for the rear (or possibly upgrade to the N803 for the front). Anyway, other B&W owners, what sub do you have with your system? Does B&W make a worthy sub, or should I look to something else? My room is about 17x15, and the sub will be along the side wall, not in the corner. Thanks.
argent
Thanks all for the advice. I am trying out the REL Strata III right now. It is a great sub, and I am tinkering with room placement. I might keep my M&K sub and use it in conjunction with the REL for home theater and then just use the REL alone for music listening. Probably not a good idea, using two different types of subs, but for home theater I might be able to live with it.
For home theater, it is FINE to use two different types of subs - for two channel it is a mistake.
Will match well. If the Sub Utopia is what you like...email andre-p@e-speakers.com tell him aditya sent you...and tell him you'd like a mono sub using a Focal 33Wx or a Focal 38Wx...he will custom build a crossover and amp for less than a $1K (my guess)
This should outdo a Rel (too shaky, IMO) if you need low rumbles at low volumes with clarity go with focal
(i also realize audiogoner love Rel...so i'm going to get badly pelted)