Hi, and welcome! I can't help you much on the B&W, as there's a bit of a feeling i've got towards brick and mortar brands overcharging for subwoofers, but I will recommend considering the Rythmik F12 subwoofer. I know the idea of a 12 sounds intimidating, but at low volumes it won't "shake the room" any more than an 8, and more importantly, will have deeper frequency extension (covering pretty much the whole audible spectrum), better dynamics, and a servo-control of distortion. Even a 21" subwoofer can be tamed for quiet listening, so never thing that a sub is too small. The bigger a sub and its motor is, the more easily it can produce low frequency sound (although upper frequencies slowly begin to roll off when you get into 18s and 21s but they're still outside of the operating range.
In general, the audible human spectrum for is around 20hz - around 15,000hz (i can just barely hear 16,000 hz tones but I'm still young!) We can feel/perceive, although not hear, sounds below 20hz, and i'm pretty sure the lowest note on a Pipe Organ comes in at a whopping 16hz! Not sure if you listen to Organ music though, haha! In general, for movies we like extension down to 20hz, and for most music, a sub that can play down to 27hz or so will give you pretty much the whole musical spectrum.
While a ported 8" subwoofer can sometimes extend down to these frequencies at times, it's usually designed moreso for upper bass and may have audible shortcomings - and while I don't know much about the B&W to be decisive, I really doubt it's capable enough in the lower frequencies to really qualify as what I call a "sub"-woofer unless it's use in a room much smaller than yours with dependance on room-gain. Look into 10" subs as a bare minimum, but 12" subs are a worthwhile improvement. Boomyness is often a function of poor design.
Now, if ~850 dollars is high on the budget (i'm not sure what the MSRP of the B&W is supposed to be.. it would help if you would give a rough budget), there's certainly other options to consider. Another suggestion I've got is to pair up two lesser subs as this will improve in-room response smoothness at different seating locations.
The only caveat is that you're buying it based on a 30 day in-home trial period, which some consider to be a "blind buy". I still suggest looking into internet direct subs; the Aperion 8d, 10d, Emotiva Ultra 12, and the Epik Legend. They offer a much better value-to-performance ratio... those subs I named would probably retail for twice their listed prices if sold at a regular store. Those companies have fine customer service and support, so have no concerns on that regard.
Now if your location affects the availability of those internet direct brands, It would help if you would elaborate.
One final suggestion I've got is to take that subwoofer you already own and try changing its in-room placement and level adjustments. It may just be incorporated wrongly.
