Diffusors behind dipole/planar loudspeakers are preferred to absorbers by many, though not all, owners of such speakers, for a couple of reasons. Too much absorption can result in a room overly "dead", but that’s true no matter what the speaker design. But since dipoles put out as much sound from the rear as from the front, diffusors behind them creates more depth (detractors claim it is false depth added to recordings containing no such depth), and a "bigger" sound, which is one thing planar lovers like about them.
@auxinput makes a good point about the distance required for diffusors. Not only between the absorber and the speaker, but between it and the listener. A couple feet to the speaker and 6’ to the listener, minimum, is a good rule-of-thumb. Do call GIK, they will provide you with excellent advice. I’ll soon be installing a set of their Gridfusers on the wall behind my planars.