I think that part of the issue is that your front speakers are both too close to the back wall, and also too close to the side walls. I realize that the side wall issue is sort of forced on you by the size of your screen.
To avoid boominess in the bass, Def techs typically need to be at least a foot in front of the rear walls (and probably more like 18"), and also (ideally) should be a couple/several feet in from side walls. Given the confines of the room arrangement, you might be happier with non-bipolar speakers of some kind.
I don't know how much money you have invested in your screen, but another option might be to go to a smaller screen, to let you move the speakers in a bit more to the center. if nothing else, moving the speakers a little more forward, away from the back wall, would help. You might also want to cut the bass a bit with your tone controls, or else set the front speakers to small, so that everything below 60 or 80 Hz. is getting sent to your sub.
If all else fails, putting some acoustic room treatment pads on the side walls next to the front speakers (while still moving them forward) would tend to damp out some of the excess reflections.
Experimenting with other sub placements may also offer some help.
As others have noted, DefTech bipolar speakers are sensitive to room placement. But, conversely, once they are placed well and set up, they are INsensitive to where you sit in the room.
Good luck. I also have a DefTech set-up that required a lot of playing around, due to a corner placement of the TV, and the fact that there wasn't a full wall behind the right-front speaker. I placed it in front of a decorative pillar, which serves as a substititute rear wall.