Get that center up off of the surface of the stand. Like 1-2". Right now, ~half the sound is reflecting off the cabinet and going straight up. Pull the 70s out from the wall a few inches. The Cambridge subs weren't bad, but you can definitely do better. That Cambridge sub I think was supposed to come as a pair originally as well. 1 big sub would be fine in this room and the room looks to be a good shape for one to work well, but it is likely to be about the size of that Cambridge and 2x as deep. So, 2 smaller ones might be easier to fit in.
As long as you don't block the driver fully (unless you got a down firing model), it would not matter if you put the sub next to the couch, however there's no saying that is a good place to actually put it in that room. An SVS cylinder sub has a small footprint, but is tall, it might fit the bill here.
https://www.svsound.com/products/pc-2000-pro