Try adjusting toe-in and rear/side wall distance to determine the optimal placement for soundstaging prior to using any acoustic treatment. You may want to try diffusion on the wall behind the speakers. Auralex makes several affordable alternatives. This may create a more spacious image. Absorption will probably result in excess bass unless full-range absorption is used and that is usually very bulky and expensive. Acoustic treatments directly on the rear of the drivers will change their physical operation in unpredictable ways and will almost certainly degrade performance. Check with Eminent Technology before venturing down that path. Good luck!
