While there is no one-way-fits-all solution, i think it is safe to say that most folks use diffusion behind their speakers. Some audiophiles, with dipoles for example, might treat otherwise - so generally speaking here.
Absorption may be used, however, if the room suffers from slap-echo problems and you are limited to where you can place treatment.
Absorption may be used, however, if the room suffers from slap-echo problems and you are limited to where you can place treatment.