What material is the wall? Is it concrete/brick or sheet rock? I am in no way an expert, but I subscribe to the view that diffusion is better than absorption. I would try diffusing the side walls first. If you still have excessive upper energy, than try absoprtion. But if you have concrete walls, yeah, absorption would probably work better.
With absorption, you could end up "absorbing" some of the good high frequencies as well.
FrankC
With absorption, you could end up "absorbing" some of the good high frequencies as well.
FrankC