Ore layer,
You mention ahaving a flutter echo problem. If you have a carpeted floor between the speakers and listening chair, then the flutter echoes may not be ceiling related. Do you have a carpet?
Personally, I have ceiling diffusion from the front of the speakers to the listening chair and 8 feet in width. Above the listening chair is a bass trap to reduce a height modal null. Of course your room is different but first start with covering 1st reflection points and them test out directly above your head.
I would be surprised if having the ceiling diffusion at different heights on your sloping ceiling makes an audible difference as the sitting distance is not near field.
You mention ahaving a flutter echo problem. If you have a carpeted floor between the speakers and listening chair, then the flutter echoes may not be ceiling related. Do you have a carpet?
Personally, I have ceiling diffusion from the front of the speakers to the listening chair and 8 feet in width. Above the listening chair is a bass trap to reduce a height modal null. Of course your room is different but first start with covering 1st reflection points and them test out directly above your head.
I would be surprised if having the ceiling diffusion at different heights on your sloping ceiling makes an audible difference as the sitting distance is not near field.