Learning about diffusion, absorption, diffraction teqniques and optimizing them is a science. Literally a science- that few people really understand very well.
You'd be best to start reading the acoustical master thread. Hiring a pro to come measure your speakers, room and develop a plan is the optimum coarse of action. If that's not possible I'd stick with basic "value" products and techniques that usually deliver consistent results. It won't be perfect but it should help a lot.
"Too much is almost enough. Anything in life worth doing is worth overdoing. Moderation is for cowards."