Sidewalls are easy to ignore. In fact, I'll bet you haven't a clear memory of what they even look like or their distance from the center seat.
Most of us know our front and rear walls. We stare at or pass through them each time we enter or leave the room.
Yet, in spite of our sidewall attention deficit, it is they that can impart an unwelcome sonic signature.
At AXPONA it was our treatment of the sidewalls that helped bring a smile to so many faces and compliments of excellent imaging. "Thank goodness you guys get the idea of soundstage."
Years ago I thought there was but one formula for treating sidewall problems: find and discourage the point of first reflection. In those days we armed assistants with mirrors to pinpoint the exact spot.
Today we understand no two speaker types are the same. Where to place a sidewall diffuser depends on a great number of parameters and they rarely are best in a standard place. In Chicago, that place turned out to be quite near the speakers. At last year's RMAF it was closer to the listener.
My best advice sounds like a broken record. Use your ears to place sidewall diffusers.