You don't need a sub to "match" your speakers, you just need a good sub. No, I probably wouldn't buy a B&W sub either, especially not for that price. For that price you could get TWO excellent subs.
Take a look at the typical recommendations: Rythmik F12, Epik Empire or Legend, Emotiva X-Ref12, HSU VTF-3, SVS, etc... ANY of those should do well in a room that size.