Maybe it so simple (and difficult) as speaker placement? The BL20 is a full range speaker, but the lowest frequency may not be possible to hear in your listening position.. Thats the great thing with subwoofers, they can be placed where the bass fills the room (or your listening position) the best. While the speakers is placed for best imaging and stereo seperation.
Just for a test – try to use the subwoofer-trick to place one of your BL20. Place it your listening position and walk around the room and find where the bass is best – then put the speaker at that spot.