@orthomead - Your problem may not be the "reflection point"
High frequenciy issues often occur at ceiling level - which is why you often see either large disks or cylinders hanging from the ceiling in large venues
- I had this problem and it was rectified by hanging a piece of vinyl about 20" deep x 8ft long behind the listening position
- right against the wall and butted up to the ceiling
- the vinyl is very good at absorbing sound - it converts the vibrations into tiny amounts of heat, which is dispersed naturally
- I finally used a vinyl roller blind and extended it about 20 inches
- I did try longer, but it made no difference
- I have also tried this at a couple of other residences and had the same great results
I started this experiment with was a piece of vinyl I had from a company that makes flexible signs - it is white and about 1mm think,and they will have the length you require (I suggest the width of the room)
You could try placing the "clouds" butted up to the ceiling (against the wall) to see if that helps, but I do recommend the vinyl approach
Hope that helps - Steve