Do you sit in the seat in the bottom center of the image? If so, how close is it to the rear wall behind the seat? ,...ie, ears to wall in distance. You may benefit greatly from thick absorption there, if this is the case.
All the typical caveats apply; whatever bass traps you can get away with, mind the early/first reflections with absorption if needed.
Here's some great vids and reading to help out, these are great;
How to set up a room here
RealTraps acoustics basics found here
, is a simple and solid place to start.
Real Traps founder/owner and fellow AVS'er, Ethan Winer, has a tutorial page
it is comprehensive.
Ethan has placed some outstanding videos
on his site, the second one is of particular interst in this case.
It's not easy to tell from your image, however you may benefit from bringing your speakers in somewhat from the corners, and off the front wall a bit. Don't worry about narrowing the spread, getting them away from boundaries often increases the smoothness and image quality. Experiment, check out the links I provided, they're very good information. Good sound is all about the room. Regardless how expensive the gear is, it'll sound bad in a bad sounding room. The room dominates the sound.
Best of luck