Diffusion or Absorption behind the speakers?

I've experimented with both diffusion and absorption to no end. Absorption for bass, tube traps in corners an absolute necessity. As far as the mids and highs, I've been all over the place, absorption alone, diffusion alone, and a combination of both. Currently I'm using only absorption (only on middle of front wall), as it seems best with current state of system.

I've found that using diffusion/aborption or a combination of the two mostly affects soundstaging in my system and room. Diffusion brings soundstage forward, absorption rearward, I change treatments in concert with changes in my system.