That article that Jack linked to explained what I was referring to.
In reply to:
I did hear that there is some sort of high end roll off (or enhancement, maybe?) specified by the THX standard.
"Re-Equalization generates a slight roll-off of the high-frequencies in movie audio to compensate for an artificially boosted high frequency used due to movie theater acoustics. Movie theaters roll-off the highs slightly due their design so a little high frequency signal boost is applied to compensate, however, that boost is not need in a home theater resulting in an overly bright sound."
I think this is the only audible difference between THX and DD.
