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.