Diffusers: under what circumstances might they help when placed behind speakers?

For soundstage depth improvement, try moving the speakers out into the room more. Also removing furniture and gear from between the speakers or get it as low to the ground as possible will have a huge impact.

OP:  I think you hit it on the head.  Generally speaking, acoustic treatments make things better in the dimension you add them, so if you want depth, diffusion between and behind the speakers is good, as is behind the listening location. 

Also, don't underestimate the value of absorbers on the ceiling between listener and speakers.

Diffusion are you ready? Maybe this will help see here and here!

Mike