Hum Issues - Rega P-25 w/ Grado Reference Platnum

My friend just put a Grado Reference Platinum cartridge on his Rega P-25. As the cartridge tracks toward the center of the records, the hum increases from non-existant to unacceptable.

Is there anything that can be done about this?

Is it possible to put some type of shielding between the motor and the platter? Or perhaps coat the underside of the platter with a copper-based paint to shield the cart from the inductive interference from the motor?

Has anyone else experienced this?

Thoughts?