One word: bitrate. If these discs use the standard 192 kbps rate for Dolby 2.0, it's going to sound anemic in comparison to a 5.1 track (which is the case with the 2004 discs). If they use a 384 kbps rate, the sound will be fuller. Ideally, THX would use a PCM track instead, which has a bitrate of over 1500 kbps, but I doubt they'll do this. I don't see them using a 4.1 track to approximate the 70mm experience.