Like Randy I’m wondering why you need to treat behind the false wall.
My screen “wall” only extends about 1/3 way across that end of the room the rest opens up into the dinning area. Rather than build a false wall I’m just making a wooden frame to hang the screen from. I don’t even plan to drywall it.
Now this is based on being able to pull my speakers out 3 1/2 feet or more from screen so even though there will be a wall close behind the right main there won’t be one for more than 10 feet behind the left and center but the soundstage is not effected.
More important in my room as Randy suggests are the corners (bass) and FRZs (front soundstage).
