It does sound as though you have some kind of ground loop, but without seeing your setup it is hard to be sure.
There is a tiny tiny chance that a hardware fault in the H9 is creating your hum, but you say that "the issue does not occur when I use a separate supply for the H9" – this means that the H9 is not at fault.
The above confirms to me that you have a ground loop, probably through your common power supply. You should either remove any superfluous grounds, or if you cannot, run the H9 on its own separate power supply.
