First question. Are these QRD (Quadratic Residue Diffuser) diffusors? If so, then the placement and direction will definitely affect acoustics in different ways. When I say "direction" I mean the placement of where the shallow slots are vs. the deep slots - also vertical or horizontal. It the room is huge, such as 20 feet by 20 feet, then it may not matter much (unless the diffusors are placed in the corners).
There is no hard and fast rule on placement. I would probably say that you should not place the close to or behind the speakers because they can actually cause phasing problems with certain frequencies the speakers are producing. They can be placed together or you can split them apart by a couple of feet. Do you have the ability to temporarily mount them and listen to the result? This may be an exercise in experimentation before you decide on the best result. It was certainly this way for me when I added QRD diffusors to my room.