The Welti approach mentioned earlier is different from most other methods since it makes no attempt (zero, zip, nada) to improve the frequency response of the bass, just minimize spatial variance. The recommendations of the Welti paper
(four subs: one in each corner or one at the midpoint of each wall) are intended to give you the same bass across all your seats. Doesn't matter if the frequency response is crappy, as long as it is equally crappy across all seats. (In practice, Welti's subwoofer placement recommendations will make the frequency response smoother, even though that is not his intention.)
One of the biggest issues with equalizing bass response is that fixing a problem in one seat can result in worse response elsewhere. Welti figured out a way to minimize that by finding subwoofer locations that yield the most consistent response across multiple seatings. Once this is done, you can EQ knowing that fixing problems at one location will likely fix them everywhere.
Which brings up some caveats: Welti only tested 2D (all of the subwoofer locations were on the floor), only tested rectangular rooms (which the OP doesn't have), and equalization is mandatory to shape/smoothen the frequency response. IF you're setting up a room with a single listening position, then you're better off using the Geddes approach
rather than Welti's method. IF you're concerned with all the seats on your couch, then it's a good idea to look into Welti's approach or the double bass array mentioned by Paul (especially useful if you're not sure of the final seating locations).