I posted this in your headshell thread....
Not sure what caused it, but on on my SL 1200gr with the MP-500 there was a hum issue with every metal headshell that I tried (including the stock one). I tried a wooden headshell (bought on Amazon) and that solved it. Probably could have been solved with non-metallic screws instead.
I should add that none of my other cartridges have the hum on the same turntable.

