"I've found that the cable connection to the H8000 is loose at best. They never seem to lock in very tightly. "
I, too, have found the adat connectors on the H8000fw to be quite problematic with 4 or 5 different brands of adat cable and that it is this loose connection rather than any wordclock issues that cause audio sync problems.
I would like to repeat the question of recommending an Adat cable brand. I have approximately 10 devices that use Adat and the H8k is the least secure.
My current DIY fix is adhesive putty!