Add a sub to BW N804?

Try a REL Strata III or Storm III subwoofer (depending on room size and budget) and you'll be amazed at the results. These subs integrate very well and will not only greatly improve the low end but also give the impression of cleaner and more open mids.

The B&W ASW 2500 is an equally good choice. the RELs are superb subwoffers; the ASW2500 is built to blend with the Nautilus series.

Believe it or not I found great results in adding ASW800's to my 801 III's and never looked back. I tried many sub woofers even the newer B&W ones and nothing seemed to blend properly. Nothing at all against REL either, but I think you will find the best match staying with the manufacturer and sub model current with your loudspeakers. Hopefully with similar materials for drivers and such you will experience what I did and never want to seperate them again. Mine also run this way for video and what an experience that I added one for center to be full range too. Remember that all loudspeakers begin to roll off around 50Hz anyway and that adding stereo sub woofers are awsome if they are placed and matched right. Good luck and happy listening.