According to your graph you could use some support below 60hz. My experience has been that 2 smaller subs work better than 1 large one. I have a simple rectangular room 12.5 x 19ft with 3Pi's in 2 corners and 2 15" sealed subs in the corners behind me. I have no problem getting good integration in that setup. I previously had 1 large 2 driver sub and it was not as easy to integrate and always seemed to take more power to get good bass at the listening position.