Several questions:
How far are you from the face of the speakers?
You say,"When seated in the sweet spot, all the highs seem to be comming from where the wall meets the ceiling."
That would be 'behind' the speakers at the joining of the 'rear wall' is that right?
If so, that strikes me as odd.
I think, to start, I'd try listening, more nearfield...as a matter of self defense.
Next experiment, this might sound really dumb, but, note how some concert hall, stage areas use vertical hanging's from the upper part of the stage.
Try, for a point of reference, hanging something absorptive from the ceiling (a towel for experimentation, if nothing else).
Put that hanging, parallel to the rear wall--starting a foot from the face of the speakers, then further and so on.
Maybe a 'team' sports banner would help a bit, but you've not got much room--I know I couldn't walk in that room with anything more than 5" or so hanging down.
This is very approximate, but we don't know enough yet.
If you'd like to discuss this, you can write me at my personal email, lrsky@insightbb.com. I'll be glad to give my phone number to talk more.
We can solve this somehow...no accoustic problem is too much when you're willing to experiment.
Good listening,
Larry
