There is nothing to do. It is system dependent and if yours hums, live with it or change the cartridge
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It could be a ground loop or incompletely grounded tonearm cable. Check your ground lead on the tonearm cable first. If it still hums, try lifting one of the ground leads from the cartridge. Your Music Hall TT might also have the same problems as rega TTs have with Grados due to magnetic interence from the motor.
While checking for ground loops or other grounding problems is worthwhile, Grados often hum if operated within the range of the motor field.
Changing to a cartridge other than a Grado is one option but you can also look for shielding material to apply to the bottom or your platter or around your motor. Someplace there is a magazine article on modifying a Thorens or AR table to allow use of a Grado but I don't recall where.
This is one disadvantage of a 'table with an inboard motor. I recall that Grado carts are particularly prone to hum via motor interaction - a problem noted by some users of the Oracle 'tables. If this is the problem (it could also be related to grounding issues as thsalmon notes) then a new cart might help, but a table with an outboard motor will put an end to the issue for good.
I have used Grado Blue with my vintage REGA P2. It is an older turn table model with ACOS S-tonearm and detachable headshell. I have never heard any hum form Grado cartridge.
My guess is not all REGA models have the same shielding/grounding.
This turn table came with a separate ground wire for the PHONO amp, perhaps it made a difference.
Most tone arms with a single-ended output will also have a ground wire for the tone arm ground.
In the case of a high output cartridge like the Grado, sometimes the cartridge can pick up hum from the motor if the motor is not also grounded to the arm. This is something that has to be added in some cases, like my example of the Rega above.
The wire was added to the case of the motor and went to the base of the tone arm. It was a bit of a trick because the metal case was an alloy that really didn't want to be soldered. I filed off some of the plating and used a good solder flux to assist with the soldering. This took the Rega/Grado combination from having an annoying background hum (usually only audible between cuts) to no hum at all.
This may be something that you will have to get a technician to perform, and I can't say it will work in all cases, but it is certainly worth a try!
wow atmasphere! thanks! that does sound like something i'd have done. i appreciate everyone's feedback and FWIW i had another audiophool friend come over and he thought the hum was minimal to naught...
enjoying the grado
ps. i also picked up a vintage magnavox console radio. it has phono (sans actual tunrtable), am, shortwave, and fm. i took all the tubes out of the AM and cleaned it up, but someday i'll have to take it all apart and give it a nice polishing.