Excellent info here, guys. NOW I know what the diffusers I saw Danny using at LSAF a few months ago are and where they came from!
I built the leanfuser stepped diffuser back around ‘14...this was done with birch ply and arranged in what Tim referred to as a “7-unit modulated array”. I’ve built 2D QRD well-type diffusers and 3D skyline-type as well and the stepped diffuser is a few orders of magnitude easier than both. Like a few of you in this thread, I left a 4” air gap behind the array and filled it with rockwool. Sound stage depth and image resolution with the Acoustats actually got better when I thought there was no room for improvement.