Diffusion or Absorption behind the speakers?

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.

diffusion, with traps in the back corner's also.

absorption - assuming your speakers are sending acoustic energy forwards - if you have dipoles or panels and want to reduce the quarter wave cancellation (comb filtering) from reflected energy with primary energy then diffusion is also helpful.

Since all bass is omnidirectional below about 500 Hz - you always get reflections off the rear wall behind the speakers that cause quarter wave cancellation in your bass and lower midrange (peaks and nulls - looks like a comb - it makes the lower midrange sound diffuse and lacking tangibility - especially noticeable with male vocals - although some people like this effect as it makes for a bigger diffuse soundstage - the Bose 901 approach).

Since you can't really diffuse bass frequencies (wavelengths are too long) then absorption is the only option for conventional speaker set ups.