I can't speak for this Grado.I have a Silver Grado with VPI MK3 Table,and was at one time having problems with mine similar to yours,and had to use a better shielded toenarm cable straight din to RCA onto the phono of preamp.
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Grado's are notorious for the poor shielding of the cartridge, which in your case is picking up hum from the TT's motor. Used to be a problem for me until I got a TT with a well shielded motor. I've seen suggestions in the past for shielding motors. Perhaps some one will chime in with some suggestions.
Is the motor on your Wilson turntable directly underneath the platter? What I have seen some do in the past is to place a thin sheet of aluminum foil underneath the turntable mat. This will create a shield in-between the cartridge and the EMF fields being created by the motor.
This is ineffective if the motor of off to the side. But if it is directly underneath, it may be an inexpensive way to create some shielding without breaking the bank.
Had the same experience with Grado's when I used them many years ago. I guess they still choose to not include shielding. I believe lead creates a good shield, but is obviously heavy. I wonder if you could address the shielding at the platter and or mat. There was a company called Merril that used to make mods for the classic AR turntables, one of which was a motor shield since those motors were unshielded and really did a number on the Grado's. The metal plate they sold to shield the motor attached to the plinth and did offer some improvement, but my Grado still had quite a bit of hum even with the plate. I also swapped out my motor with the Merril modded motor but don't recall that helping as much as the plate did. Otherwise I always liked the Grado cartridges. I had three as I recall, the best of which was in the signature series. All MM cartridges. On the same table I tried a Sumiko MM and got no hum at all.
PS A quick search on magnetic field shielding (MU shielding) turned up This Site. I'm sure you can find others.