Far from an expert, I put 2 inch 703 on the backside of my side diffusors in the aim to absorb anything that might make it that far. It probably was not very effective and I am too lazy to measure with and w/o it and then try and interpret the sweep.
My bang for buck came with side diffusion. When that went into place I really liked the result. I was fairly close to the back wall and so generally everyone who is can benefit from diffusion behind you. I had rear side diffusion and then some back wall diffusion and then right behind me diffusion but I did not notice any improvements although maybe a sweep would have revealed some change. Again, too lazy. I added ceiling diffusion and that helped some more. So it got me to thinking that the side walls being so close to the speakers could work and walla, that made the room feel larger acoustically - more hangtime in the notes, more spacial feel. Not like an auditorium and some magical transformation, but noticeable improvement in soundstage width.
Of course now with 8's and no fill, the speakers can completely disappear with the right recording, proper electrons, and if I hold my mouth just right.
