You've eliminated a lot of stuff but you're missing something. Bias may read fine thru sims but have you checked the contacts for the drum & DV? That would make sense to me since you've replaced with new drum & DV units but still getting same results.
EDIT: Use Allan's idea. Stop the print process in time to inspect the drum to see if the image is nice and dark on the drum. If not, there's a good chance you don't have good drum charge or high DV bias voltage.
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