My experience with Quad ESL 63's might be slighly helpful, in that it tends to confirm your main intuition about total absorption of the backwave. At first I built DIY Styrofoam diffusors imitating the RPG Omniffusor, and placed them on stands parallel to the back wall and about 2.5' on average behind the toed-in Quads. That was a big improvement over having nothing behind the Quads. Then I became persuaded that absorption, not diffusion, was the way to go with this backwave, and I filled in the diffusor wells with polyester fiberfil (from a fabric store), and that improved the sound quite a bit. I haven't gone further than that because as far as my ears can tell, I don't have to. I also don't know quite how I'd proceed if I wanted still more absorption, without messing up the looks of the room, making it hard to get at windows, and so on--annoying practical difficulties.
When I believed in diffusion, I once advised a Martin Logan owner to try Argent Room Lenses behind his. I tried them behind my Quads, for which they're a wrong shape, and they didn't work at all well. That figures, given the wrong shape and the fact that absorption, not diffusion, is wanted. Further, the Room Lenses don't absorb at all, while my DIY Styrofoam diffusors--design info supplied on request, and on Room Lens clones too, by t he way--definitely do.
When I believed in diffusion, I once advised a Martin Logan owner to try Argent Room Lenses behind his. I tried them behind my Quads, for which they're a wrong shape, and they didn't work at all well. That figures, given the wrong shape and the fact that absorption, not diffusion, is wanted. Further, the Room Lenses don't absorb at all, while my DIY Styrofoam diffusors--design info supplied on request, and on Room Lens clones too, by t he way--definitely do.