Diffusion or Absorption behind the speakers?

What is a good way treating the wall behind the speakers?
I see some people using only diffusion, some use only absorption and some people using both.
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.
Agree with Tiger. I found diffusion on the front wall between my speakers gave me a very ‘alive’ and vibrant sound, yet still detailed, accurate and natural. It also seemed to increase my soundstage. Both corners behind the speakers I have DIY bass traps (R19 rolls)

Absorption on front wall between the speakers tended to make things a little dull.

I've experimented with both diffusion and absorption to no end. Absorption for bass, tube traps in corners an absolute necessity. As far as the mids and highs, I've been all over the place, absorption alone, diffusion alone, and a combination of both. Currently I'm using only absorption (only on middle of front wall), as it seems best with current state of system.

I've found that using diffusion/aborption or a combination of the two mostly affects soundstaging in my system and room. Diffusion brings soundstage forward, absorption rearward, I change treatments in concert with changes in my system.
I would also second Shadorne thoughts diffusion seems to work best in a very large room . Bass trap's are what most people need in normal size rooms only after you address this should you think about diffusion. I would also address the first reflection point with some absorption before adding diffusion unless the room is very large. I only use absorption in My 23 foot by 17 foot room a nice size room and My speakers have a rear tweeter . I have the speakers about 5 feet out from the wall and have a trap on the wall between the speakers along with every corner and first reflection points plus a couple of others. Marc