That is an extremely large volume to fill for one sub. I'm guessing you're close to 15K^ft3 of volume with just the viewing area, the kitchen, and the room behind the couch. Unless you close the doors, you need to include the volume of those rooms as well. I'm assuming the space behind the couch is the second 17' x 18' area you were talking about with the 20' ceiling. You're already corner loading the sub so no more SPL can be gained from that. I could be you are sitting in a null. I would highly recommend doing the sub crawl before anything else.