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

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

Mike

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.

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.

Thanks Erik.

I have no complaints relating to clarity, but am thinking more along the lines possibly enhancing the depth of the soundstage.

A diffusor placed centrally and behind the speakers is very often done in high end showrooms and private listening rooms.  Adds clarity without deadening the mid-treble range.