I have a 48x56" Rockwool sound absorption panel installed into the wall behind my listening chair. Covered by a print of the Grand Tetons taken at 10,000' .
In another room I was forced to vacate I had a closet the length of the wall behind. I put absorption on back wall of it and removed the doors. That extended the size of the room and the distance behind the listening chair. It was probably the best sounding dedicated room I have used.