Diffusion vs. Sound Absorption views

Agreed, in my experience both absorption AND diffusion was needed.  However, I would look at people who have specific experience with open baffle speakers.  You can try use diffussion panels behind the speaker, but make sure they are generic diffusion, such as an array of pyramids or a convex diffuser such as the GIK Evolution PolyFusor.  I would highly recommend NOT using a quadratic/QRD diffusor right behind the speaker.  The QRD diffusors can do weird things to the phasing of sound waves if they are so close to the speaker or the listener.  A QRD diffusor could work well on the front wall in between the speakers (as long as there is good distance away from the speakers) or on the back wall.  However, I have found that each room is different and I had to experiment a LOT with placing QRD diffusors.  Many locations of the QRD diffusor made the sound worse. 

I will repeat my warning and caution about using QRD quadratic diffusors.  snapsc had a point about diffusors " out of phase wave interfering with the sound".  The QRD diffusors can definitely cause problems here if they are placed in certain spots.  I have had situations where some of the midrange frequencies were boosted and others were cut, making the sound completely terrible.  The QRD diffusors can also boost high frequencies to the point where it's too bright/harsh sounding.  These side affects are entirely dependant on where you put the QRD diffusor and the specific room you're operating in, so it becomes much of a test and listen exercise.