Hmmm..."big" sub? You haven't been around here much, have you?
For optimum sub placement, one must perform the "sub crawl". Place your sub in your main listening position, put on some bass heavy music and crawl around on the floor until you find the spot with the most/cleanest bass. That is where to put your sub.
Also, if your measurements are correct, and you have an 8' ceiling, that's a 4800 cubic foot space. One 12" sub doesn't have a prayer of pressurizing that large an area. Head on over to the Subwoofer sub-forum to get more advice on what it would take.
Your trouble could be coming from the seating position. Either move the sofa off the wall some, or even better would be to put the TV on the 15' wall and have the sofa with its back towards the open kitchen.
Having a long-wall setup like you have with the seating position hard up against a wall will be creating lots of peaks and nulls in the frequency response.