Why not try a pair of the Magnepan DWM woofers. These are not subwoofers per se, but will add more panel area and will really smooth out that bottom end and it will seem like you added subs.
I heard a pair of the 20.7s with (2) of these and it was superb!
I heard a pair of the 20.7s with (2) of these and it was superb!