If your imaging is horrific, I wonder how you have your side/ceiling clouds placed. They should be at the first reflection points. One trick for this is to have a helper move a mirror along the walls and/or ceiling while you are sitting at the mix position. Anywhere you can see the relfection of the speakers in the mirror, put your absorbers there. For more on this, see Creating a Reflection-Free Zone
Also, in addition to Glenn's good advice about treating the rear wall, I'd also get some good corner absorbers in all the wall/ceiling corners, and even the wall/floor corners where practical (ie, behind the speakers).
Diffusion.... could work. But it will only work well after you get enough absorption in the room.