FWIW, I just got a subwoofer to integrate with my B&W N804s. I think you have to take room size and room treatment into consideration when considering a sub. Also, consider whether you'll be listening mostly to music (sealed sub) or using it mostly for HT (ported sub). I run my speakers full range and then have the sub cross-over to fill in the low bass. It has really improved the sound of my system dramatically. Based on the specs of the N805d3s, I would guess that your cross-over would be set somewhere around 50Hz, but you'll have to play around with cross-over, phase delay and volume in order to integrate the sub seamlessly. It can be a bit of a PIA, but once you've got the right settings, it's glorious. If you do get a sub, I would suggest getting one from either Rythmik, Hsu, SVS, REL, or JL. I bought a Rythmik L12 and am a very happy camper. Keep us posted on your progress.
