Grounding the unit to other than the phono stage may help? Ground it to your pre or even to a screw on the plate cover on the wall plug?
Are you using a high gain cartridge with a low gain input? Is this hum as soon you you switch to phono on the pre or only when a record is playing? We need more details to help narrow down the problem.
Are you using a high gain cartridge with a low gain input? Is this hum as soon you you switch to phono on the pre or only when a record is playing? We need more details to help narrow down the problem.