Hmmm, thats very starnge looking man?!?!?! I noticed you said you ran channels 3/4 LINE-IN, Phantom ON. Why did you run LINE-IN with Phantom ON?!?! Is that what people do with their 680s?
Like you said, it goes from fine levels, to BAM, total weirdness. I honestly have no clue other than I do agree with you, and think its a Phantom thing too. Just have no idea WHAT its doing?!?!?!