Ceiling diffusion

Thanks for the responses. I will start with first reflection points and experiment from there.

When facing my speakers from my listening chair my ceiling slants up from right to left from 7.8ft to a little over 10 feet. Should the diffusion/absorbtion panels be mounted flush on the ceiling or hung so they are parallel to the floor?

Kevinzoe,
I have Audio art 2" panels on the ceiling taking care of 1st reflection points and 4" panels on the 1st reflection points on the walls. I installed 4 DR6 (MSR Acoustics) diffusion panels above and slightly behind my listening position. You are correct, I did not hear any difference.
Yes I have carpet. After some careful listening I think the flutter is coming from two adjecent rooms that are open to the listening room. Maybe I should try treating those two rooms as well.