FYI: The Shure has a ground tab on the right channel ( green pin). Sounds like you now have a grounding problem. If you have the tonearm ground lead hooked up , leave it off and see if the hum goes away. If not try to hook it up to another ground source.
Shelter 201 hum
I just installed a Shelter 201 on my SME 3012/Garrard 401 combo. Disappointed there is a far amount of hum present. I just removed a Shure Type V which had no hum at all so nothing was changed but the cartridge. I checked all leads and everything looks/feels fine. Any ideas out there?