Owens Corning makes an absorbing material called 6 pound fiberglass acoustic. I am investigating this right now for my room. It cuts the first order reflections only at the uppermost frequencies, reduces slap echo, and yet does not destroy ambience. This material is much denser and more solid than what you would generally think fiberglass to be. It is applied in one, two, four and six inch applications, and is often employed in recording studios in the thicker version. For a listening room, a one or two inch covering over the sheetrock, then covered with an acoustically transparent cloth, makes a fabric wall that is attractive and acoustically correct. It is also possible this way to still mount pictures and other wall decorations, where ultra soft coverings could make this difficult. I am also looking into a fabric mounting system that uses tracks, and the cloth is rolled into a track with a tool that looks like a pizza cutter. This provides a razor sharp line where the two materials must meet, it is removable if damaged or sun faded, and does not need the additional expense of welting, sewing and staples. For the corners, behind the speakers, I plan to build a false wall, about 8" deep, with the same cloth covering it. Its false front will be removable, and then I can experiment with RPG diffusers, extra thick 6 pound fiberglass, or other treatments, AFTER all the rest of the equipment is in place. This will also hide my Tube Traps that go in the corners. I know already I want to keep them.