I have a KEF KF92 subwoofer in a room similar to yours. I also got their remote transmitter for it so I can locate it where it sounds best, which for me ended up being in the middle of the room, behind the sofa. This has different curve settings for different locations in a room: Corner, wall, room. Good starting points.
This is my third sub, and I am very happy with it compared to the others. I see that someone recommended the SVS 3000 Micro. I had a pair of them for a short while, then sold them. They didn't go very low in my room and I couldn't get a good setup with them. Though I will say SVS is a good company and the fit and finish of the Micros was excellent.