Hi Enthused, thanks for the diagram!
No doubt you have a difficult room.
The big issue I see here is that the listening position is quite close to the center of the room. It is very difficult to get good bass when sitting near the center of the room, and/or placing the subwoofer near the center of the room.
Another issue is that nearfield placement of the true sub is not very ideal for deep bass reproduction, while farfield placement of the true sub up front is not very ideal for mid-bass reproduction.
My recommendation is to move the main listening position away from the center of the room (if possible), and add an MBM-12 MK2 placed nearfield (and/or add a second VTF subwoofer up front placed on the opposite side of the wall compared to the first VTF sub).
We'd love to see some pictures too.
Pete - Hsu Research