https://bugs.freedesktop.org/show_bug.cgi?id=72087
--- Comment #2 from Andy Furniss adf.lists@gmail.com --- (In reply to comment #1)
Video can be found at http://youtu.be/lTUtw0onSCw and the issue is observed around 9-10 second mark.
That looks like some auto level thing like C.A.T.S - is that turned on on the Panny for that input?
Unlike Windows I don't think OSS Radeon will do anything other than full range RGB over HDMI - I would investigate the "dark" issue as perhaps setting XBMC not to scale to full range may be confusing something that's trying to automagically tweak things for you - Denon or TV - I don't know, but it's normal for 16 - 235 luma to have samples that are below/above which usually get corrected by the stretch to full range.
PS How did you add dither to xrandr?