I understand the vibration advantages of Dual Subs, and also that for the best in-room response multiple subs are the road to go. However, the article that I linked to also claims that dual opposed subs have a spatially more uniform pattern, and that is what I am after.
Below is an REW measurement of the in-room response of the Kube8b, positioned in the front left corner of my large listening room, and equalized with a DSpeaker Antimode 8033 (only for the subwoofer). I used multiple listening positions for the equalization, so response at the main listening position is not as smooth as I could have achieved by equalizing for just one position. The main speakers are Quad 2805 electrostats, high passed at 80 Hz with a fourth order filter. As you can see response reaches pretty low for such a small sub, probably due to the combination of a corner position and a large room size. The PV1d went down even lower and was effectively flat down to 14 Hz. The additional subwoofer that I am contemplating is a KEF Kube12b.