your statement:
Wiring for the tonearm to the preamp was as follows:
- Pin 1- no connection
- Pin 2- + positive
- Pin 3- - negative
- Shell ground- cable shield
is very very confusing.
This tonearm is for custom mono cartridge? For stereo cartridges, 4 color coded pins (same four pins from arm) are:
positive 1
negative 1
positive 2
negative 2
There is no ground pin for cartridge.
My experience is that ground loop hum/electro-magnetic noise has nothing to do with RCA or XLR plugs if ground wire was connected properly.
remember, Tesla proved a long long time ago that electro-magnetic energy could be transform from source (turn table/cartridge) to receiver (phono stage inputs) if no ground shelling to short the energy.