The reason for this is that THX mode probably enables the Cinema Re-EQ option, which softens the high frequency sounds a bit. Since movie soundtracks are mixed for large theaters, the high-frequency sounds are often boosted to the detail isn't lost in the large space. Many DVD releases don't remaster the soundtrack, so for some they may sound too bright. This is why the Re-EQ option wsa included in THX's bag-of-tricks.