Which subwoofer best pairs with B&W 804 diamonds?

You sound like a detail oriented individual -very precise! I recently acquired some 804 D3s which are almost broken in 100%. I'm using my old standby B&W ASW650 to satisfactory effect. Not to fussy (me) about crossover frequencies and such. I think I'm using 50hz. I just hook it up and turn it on and adjust the volume until I hear it making a difference. Really just that simple. All I'm trying to do is supplement what's already there from the 804s'. I have the 804s  about two feet from the rear wall, 3.5 feet from side wall and 9.5 feet from me.
Now if I were in the market for a new sub, I'd go with a B&W D2 (2 eights placed back-to back) or a JL Fathom 112 v2. Velodyne 12 DD would also be a good choice but they seem to have dropped off my radar lately.
Subs do make a difference with these speakers unless they are right up against the back wall. When everything dialed in they can really rock.

It depend on the room size and application, bigger room need bigger / more powerful sub, if it is for HT duty, the AV processor / receiver will do the crossover settings for you.

VANDERSTEEN 2wq model for 2-channel music along with its accessory crossovers 

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the V2W model if its an LFE output for home theatre 

Awesome thanks. Do the SVS subs, PB 16 pair well with these?

Crucial is room interaction. I have been more than pleassed with the Antimode 8033 room eq unit: http://www.theabsolutesound.com/articles/dspeaker-anti-mode-8033-dsp-subwoofer-equalizer-tas-204/
As far as I am concerned, no sub should ever be used without something like this. I believe this has a far bigger impact on SQ than the qualities of the subs themselves.