Its very odd, I downmix 7.1 to 5.1 via HDMI, but this issue only happened when I was using my analog outputs of the Oppo through the 7.1 input of my Marantz. Once I changed from DTS HD MA 7.1 to DTS HD MA 5.1 the problem was gone. The oppo has all speakers set to small (80Hz crossover in analog output) with the center trim at -.5 dB. Just wondered if anyone else experienced it, perhaps it is a mixing issue on the 7.1 track center channel.

M60, VP150, QS8, HSU VTF2-MK 3, Marantz SR8002, Oppo BDP-83SE, 50" Panasonic Plasma