Here's the first steps I would take:
Step 0: get a measurement system in place. Measure the response of each of your main speakers one at a time to get an idea of any frequency issues that may exists. Measure decay at one-octave intervals.
Step 1: install bass traps in convenient corners, particularly in the corners behind L & R, because you'll have some SBIR-related frequency issues to deal with there. Also behind the center.
Step 2: bring your wall panels down to be centered on seated ear height.
Step 3: Remeasure. You'll probably find that more low frequency absorption is in order to bring decay times into line. For your space and goals, I'd suggest a target "rt60" decay time around 0.4 seconds across the band.
Measuring for diffusion will be more complicated, but I think this should be a good start.
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