I would place the sub right behind the couch, a couple of feet away from the wall, and see if that helps. The AVS post indicated you had the sub in the back left corner; that seems problematic since the couch appears to be roughly centered in the room (front to back).
If your seating position is on the left seat of the couch (furthest from the wall), putting the sub against the wall will leave you in a null, but putting the sub too far from the wall will result in a frequency dip as well. I'm thinking maybe 2 feet from the wall would be best.
Note that with your room size and construction even a foot or two will make a big difference, so your sub crawl will have to be much more precise than normal, ie you can't stick your head close to the wall (a foot or two away so you don't bang it), hear bass, and then stick the sub right against the wall and get good results.
Good luck. You will get it worked out. You'll need multiple subs to get decent bass in all the listening positions but most of us only care about getting good bass in one seat anyways
