I ended up with one 20" in one corner and one in the center of the back wall. I put the 13"'s lateral to each main, left reflecting to front wall and right reflecting to listening position. I got a large improvement in bass "realness" that I attribute to the 20"'s. They go down to 40H.
The bass sounds more authentic, particularly the vibrato that one associates with plucked strings. Snare drums and kick drums have real snap.
