All theater compensation follows the same basic formula, be it THX or otherwise, and that is to filter out some of the high frequency. In cases where the mix was intended for a movie theater and the highs were overblown, theater compensation can help remove alleviate harshness. In other cases, it weakens teh ihgh end - creating the "deadened" sound you've experienced. Personally, I don't use theater compensation normally, so I can't speak to how the 990's theater comp compares.