I would go for the 803's, the 804's do not have any deep bass, check B&W's own specs, they are -2db at 45hz on their specs. the 803's go -3 at 35hz -6 at 28hz, which is more reasonable for a floor standing speaker. Many high end monitors have much better bass than the 804's, and also are better integrated than their 2.5 way design which is a compromise from the 3 way in the higher Nautilus series. I found the 804's to lack any magic that the higher end N series had, even though I found even the 801's lacking quite a bit compared to others in their price range.
B&W are not bad speakers, and they do hold resale exceptionally well, but there are many other and possibly better alternatives, so please, even though you are a B&W owner, listen to some other lines before you decide.
I would suggest checking out Totem, Vienna Acoustics, Audio Physic, and Von Schweikert just to name a few. All have their own sets of strengths, some very similar to B&W's.