Here is one method of reducing ground loop current sensitivity. It works because a cylindrical shield current produces no magnetic field internal to the shield. The shield from the rca jack carries loop current to the chassis star ground, and partway through this coax, I pull out the signal from the rca, and run a conductor used as the ground reference to the input of the board. The center of that coax is the only place in space where the ground loop current produces no magnetic field, so the input ground will not couple to the loop current.