Something isn't getting a ground. Use your fingers again. If touching RCA shields lowers the hum, then you're on the right track. Is there a power transformer near the tonearm? What happens if you change MC loading to 47k, does it change to just hiss? If so, then you have magnetic induction into the phono cable itself. Offending source must be moved.
Use fingers to touch all "grounded" spots, including chassis metal, tonearm, ground lugs, etc. Find out where you can make a difference.
Use fingers to touch all "grounded" spots, including chassis metal, tonearm, ground lugs, etc. Find out where you can make a difference.