Problem with soundstage up near ceiling.

This is driving me nuts. I have changed everything. The whole system and still have this issue. The room is about 14'x20'x6'8". When seated in the sweat spot, all the highs seem to be comming from where the wall meets the ceiling. I insulated the joists behind the drop ceiling tiles and tried use Owen Corning 703 in more places than I can count.
I you have ever seen this before, give me a clue.

Tim