Do you use REW or other room measurement software? The BFD won't do you much good if you don't have a frequency sweep of your room to see where the peaks and dips are.
I use a BFD combined with a 20 Hz Harrison Labs F-mod high pass filter for my subwoofer. The only downside is 20 Hz is the lowest F-mod they make so if you are looking at anything below that the F-mod won't work for you. It is cheap though, I think 20 bucks for two filters.
