John, I also thought that it muddled the highs a bit but I thought it could be my imagination since im not used to this receiver yet. If a DVD says THX on it does that matter?
I use it when for whatever reason the Dolby DD, or Even DD 2.0 or heck even Dolby PL just happens to sound harsh or brash.
You will know what I mean if you ever run across such a title / source, usually it is the dialog that is harsh, brash, overly bright. Not sure how to describe it, but everytime, the THX seems to help a lot with those. Otherwise, I use what I deem the correct mode, which will be DD or DTS.
I leave mine on Automatic surround. . . Works best for me and sounds great doing it. I got tired of the other modes. . . "Advanced" "Expanded" etc. etc. Sounds just fine doing its own thing and I dont worry about wether it is in the right mode or not. . .