Are you not using your sub for 2 channel playback?
Your issue is pretty standard for any speaker at low volume because our hearing is much less sensitive to low frequencies. As the volume increases our ability to hear bass also increases, hence the, it's sounds better when it's louder, but even at 110 dB 40 hz will sound 12 dB quieter to us than 1K hz, and at 80 dB 40 hz will seem almost 20 dB quieter. So the lower the volume you are listening at, the louder you should be running your sub in relation to your B & W's
http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/eqloud.html