I'm afraid it probably is a ground loop – plugging everything into the same socket helps, but it is no guarantee. The fact that your first H9 did the same thing suggests it is not a fault in the unit.
A test would be to replace one of the lead-H9-lead connections with just a lead (i.e. bypass). If this suddenly fixes the noise, that would confirm my view.
You probably need to have a tech look at your rig, and mess around with the grounding.