Diffusion vs. Sound Absorption views

On a basic level - high frequencies add sparkle and life to music even to those listeners with diminished high frequency hearing. With that in mind I suggest diffusion should be your first priority and when you feel you’ve done a decent job of it then limited absorption behind the speakers as you find provide enhanced detail and depth to your sound. What types of music do you listen to most?

I am keen to check out this diffusion aspect of sound treatments, as I have a remarkably dampened room (two layers of 5/8" sheetrock with Green Glue in between and several sound panels behind the speakers, and thick carpet in the room.)   I have a long narrow room (13x26x9) with the speakers on the long wall, about 3' from the wall, and I sit about 10' away on a sofa against the rear wall.  Not an ideal setup for sure, but I am working around the edges with what I have.  
If diffusion enhances the spatiality of the sound, then I am interested to explore this aspect of my system.

I listen to about 98% jazz, of the straight-ahead variety.  Miles, Coltrane, MJQ, Brubeck, Desmond and most significantly Stan Getz. 

Since you have Spatials, why not contact Clayton Shaw, owner,
and ask the source. He is a person very willing to share. Overall
I understand absorption be best used to stop early reflections,
ie in front of speakers on side walls and ceiling. Diffusion best on front and back walls.

Essentially 
Diffusion everywhere, particularly front and back walls and ceiling in your case. 
Absorption only where benefits are obvious through experimentation 

How do you know when panels are needed? I'm interested in the topic, but don't notice a problem with Golden Ear Triton 3+ about a foot from front wall and 6" from side walls. If it already sounds good don't mess with it? Tinker for better sound? Or is there some measuring equipment for this?