Depending on your workshop capabilities, I'd recommend my very own DIY imitations of RPG diffusors. Please email for my instructions for making them if you could consider this: you'd need a table saw or some other good way of cutting Styrofoam cleanly and neatly, and a work table, and THAT'S ALL! I have 23 square feet of RPG-style diffusors on my front wall, three of them added one at a time, and each one made a major improvement in the sound. My electrostatics have a strong back wave, maybe you don't have that, but the middle of the rear wall is an important surface to treat, and absorption isn't the best way to go.
