ghdprentice +1
ghdprentice is correct in that heavy absorption behind a planar speaker will overly deaden the rear wave and seriously dilute what’s great about a planar design.
Many years ago in my first apartment I bought my first real pair of speakers, Martin-Logan Sequel II’s, a hybrid design with a 48” x 12” electrostatic panel and a 10” woofer in a sealed box. My apartment was of generous size with a soaring vaulted ceiling. But I could only pull the Logans maybe 2-3’ from the wall behind them which noticeably blurred the soundstage and caused a bit of midrange glare. After some experimentation, I found some handmade tapestries in a shop on an American Indian reservation in northern Michigan. Though they were sized like a throw rug, about 5’ x 4’, they were meant as wall hangings. These were woven from knotted soft cotton roping into a strikingly original brightly colored design. The knotty texture provided some diffusion and the soft cotton offered a bit of absorption. Hanging one behind each ‘stat panel involved a simple pair of finishing nails into the wall. These original pieces of artwork brightened up the room and brought the Sequels into focus.