I'm hardly the expert here but it would seem pretty likely, based on your description, that the amp chassis is floating. If you have a multimeter try testing the continuity between the ZDT amp chassis and the power supply chassis. I would suggest doing this at a screw so that if the probe leaves a mark it won't be noticeable. If the amp chassis is indeed floating I'd talk to Craig about it. It makes me uncomfortable to think about and I don't want to speculate further.