I built a modified QRD 13 that's 20" deep, with the verticals 3.5" wide. It broadened my soundstage and helped my bass. I placed it flush with my front wall. Hemholtz absorbers may work, or Dennis Foley has plans for an absorber I'm going to try. Multiple subs will also help. I have about 20 wall panels made of 1" compressed fiberglass board covered with speaker cloth that really helped me.