Which subwoofer best pairs with B&W 804 diamonds?

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.