To simplify, the two-pin cartridge coil is inherently a balanced signal generator. On one channel map cartridge Red to XLR pin 2 and cartridge Green to XLR pin 3. On the other channel, map cartridge White to XLR pin 2 and cartridge Blue to XLR pin 3. The cable's RFI shield should float unconnected at the the arm and be grounded to XLR pin 1. A separate ground wire may be run from physical tonearm to phono stage chassis.
A standard RCA/XLR converter plug may not work, as these typically short XLR pin 3 to ground and elimininate one phase of balanced signal from circuit. To pass a fully balanced cartridge signal to the phono stage you need a converter plug that maps RCA pin from cartridge positive phase to XLR pin 2, cartridge negative phase from RCA barrel to XLR pin 3, and floats XLR pin 1 unconnected.
As suggested by Lewm, if in an RCA configuration cartridge negative phase and shield to RCA barrel share a common wire, then grounding the shield to pin 1 or chassis will have the effect of shorting out and eliminating one half of the balanced signal.
A standard RCA/XLR converter plug may not work, as these typically short XLR pin 3 to ground and elimininate one phase of balanced signal from circuit. To pass a fully balanced cartridge signal to the phono stage you need a converter plug that maps RCA pin from cartridge positive phase to XLR pin 2, cartridge negative phase from RCA barrel to XLR pin 3, and floats XLR pin 1 unconnected.
As suggested by Lewm, if in an RCA configuration cartridge negative phase and shield to RCA barrel share a common wire, then grounding the shield to pin 1 or chassis will have the effect of shorting out and eliminating one half of the balanced signal.