Have you tried a near field listening position? I am wondering if this may help reduce some of the acoustic issues you are encountering. Bring your speakers in a bit closer together and away from the rear wall a few feet. For example 4' speaker placement from the rear wall, 6' between speakers sitting about 5' from them as a starting point for evaluation.

Wow! I really like that, does anyone know where to find plans for building one?

LR, you have some cool looking stuff on there. Thanks for the link, I Had no idea diffusers were so adjustable and different. That much info hurt my brain. I know what Ill be spending my time reading about now. Do you like 2d or 3d building better? And which gives you better sonic improvement? Or are they so different they don't compare and I just didnt read enough.
That page makes me wish I had wooden legos, I would build some 3d out of them.

1D vs 2D - absorbers or diffusers, it's all just different tools for different needs.

Think of sound treatment as taking medicine to cure your frequency, timing, and reverberation ills. You really shouldn't take random medicine for ills you don't know about, right?

In general, I would say 1D diffusers make the most sense for most people. The front/back left/right walls are the ones that muck with your listening position the most - a 2d diffuser throws some sound to your floor and ceiling - is that really something you want/need? Depends on your situation.

For building - smaller 1D diffusers are a breeze to build.  Smaller 2D skyline diffusers are pretty easy to build in general. But the bigger you get, the harder the 1D builds get, whereas the 2d builds I don't find get much more difficult as they scale up...they just get heavier!

Also keep in mind, the (loose) rule of thumb is that you need 25% of your wall space covered with diffusers to be truly effective.