Center channel distortion?

[dg5150]

Here's a question: I notice in a lot of movies a small bit of what sounds like distortion in my center channel. Happens usually when a male voice is shouting. Does not occur when playing SACD/DVD-A, don't notice it as much in Dolby Digital tv signal. Seems to occur more with DTS HDMA and PCM tracks. I googled center channel distortion and found forums going back to 2006 with the same problem, multiple receivers, multiple speakers. People were also saying they went to a retailer and heard the same phenomenon on multiple setups as well. By the way, I did trade out my speaker and the same thing happened when I tested a bookshelf in the center. It also did it when before I upgraded to my Elite receiver (Pio 920 before then). I also played a frequency sweep and could hear no distortion in the center channel, so the speaker is not blown.

So what causes this? I have to believe it is the source recording. Is the HD audio so "good" that it brings out the flaws in the track as well? Just wondering your thoughts.