FWIW, I had some 6’High x 3’Wide x 12″Thick DIY free standing fiberglass-filled absorption panels behind my Majestic 845PX Soundlabs. I tried the panels on the wall directly behind the S/L’s, then across the corner and lastly on the side wall behind them. In every area, they deadened things too much for my taste and I removed them.
I built the panels many years ago, based on a Jon Risch design from the AudioAsylum Tweakers Forum. The S/L’s are pulled out approx. 7′ into the room and fairly thick, room darkening drapes are behind the speakers, essentially covering the space between both speakers.