Are you running Xvid codecs with AC3 and do you have the latest ones? There's quite a lot about audio issues with DivX so this might be related, but Xvid was supposed to iron all of those out.
You mentioned when playing DVDs via WMP is OK, but what about Xvid encoded files? Also if you play a standard (i.e legal
) DVD via PowerDVD (making sure the settings in config are set to SPDIF & Dolby Digital output) does your amp pick up the fact that it is DD then?
EDIT: You might want to have a read of this...might be relevant
http://club.cdfreaks.com/showthread....=sound+Tmpgenc