Backwall wall behind my back treatment.

Its a hard concretetype wall.

Perhaps a long pile rug ? Or a acousticmatt of pressed foam ? On the rearwall. ?

Maybe 3 inches thick of some rigid fiberglass covered in a nice fabric. Ask at the acoustics circle at audiocircle.com and hopefully Ethan Winer of Realtraps will chime in. Be sure to describe the rest of your room also and pictures would even be better.

Thanks Warnerwh ! Shall go there :)