I would consult with GIK on this. I think that wall should be third on your list behind bass traps on corners, ceiling etc, then high frequency absorption at the side wall reflections. But get some advice from the pros at GIK and Real Traps too.
Generally, if you don't have dipoles, then things like quadratric diffusers would go on the wall behind your head if your listening position is far enough away from that wall.
Generally, if you don't have dipoles, then things like quadratric diffusers would go on the wall behind your head if your listening position is far enough away from that wall.