I have the 684's with HT62 Centre. So very similar to yours but just bigger floorstanders.
I had a Rel Q200E a 12" 100W sub but it wasn't really gelling with the B&W's. So I thought I'd audition a few, tried various at Seven Oaks. More form REL, KEF, Velodyne and Paradigm at different price points. Some offered more depth and more slam than others but when listening to music I could always tell what was comeing from the sub, integration wasn't seemless.
So a week later went back and tried the B&W610 and B&W610XP. Wasn't expecting great things as they are tiny only a bit bigger than the 10" driver.
But Wow they can really dig deep and they really do gel seemlessly to the 684/685 and HT62.
I was torn between the 610 and 610XP, the XP did have a touch more slam but £200 more I wasn't sure. Seven Oaks said I could take the 610 and if I wasn't 100% satisfied I could upgrade to the XP.
Some may like the room to shake more but in my 20'x13' room I find the XP610 is a joy with music and certainly more than I require for HC viewing.
Watched Transformers 3 today and the bass accuracy and toughtness and then the slam is awesome from such a small package.
You can read what hifi's 5 star review here...
B&W ASW610 review from the experts at whathifi.com
And for another year it's won the 2011 best Subwoofer under £700 award.
I managed to get mine for £440
Cheers
Lee