Hi Randy,
The arm was (and still is) a Magnepan Unitrac. I'm not completely certain as to whether or not the arm may have played a role in the hum, but my guess is that it did not.
Although it doesn't necessarily signify anything conclusive about the possible role of the arm, I'll mention fwiw that the hum completely disappeared when the arm was moved toward the rest position, as soon as the cartridge reached the point of not being directly over the rotating platter.
The only other thing I can add is that over the years I've heard a number of other reports of Grado's having hum problems with a few various non-SOTA turntables (I don't recall which ones), from those I know as well as well as on the web.
Best regards,
-- Al