As I said in the other thread, I think you need dual subs, one on each side of the sofa as indicated in the graphic, to get reasonably balanced bass across the main viewing area. I listed four recommended subs, and I'd go with two of them. I doubt you'd ever be unhappy with any of them. So...
There are so many different philosophies regarding how to decide on subs. Your area is connected to a rather large, open space, so there will be a lot of energy loss. Meaning, you almost can't have too much sub. Personally I think you should go with the best sealed subs you can afford and fit...