Audiolab M-DAC+...RCA signal quality…….

I have the M-DAC. But the M-DACplus has the same main board, and therefore the same analogue output stage.
XLR PIN1 is not grounded internally - it is open.

From my experience, I can only recommend to use the RCA when your amplifier receives RCA input:
  • You have the right signal level
  • You have lower (half) output impedance and can run longer cables
  • Capacitance of the cable is usually lower compared with an XLR-RCA-one-side-grounded

This is valid for the MDAC(plus) without additional output stage.

My LKS with an additional stage for RCA sounds terrible compared with XLR. But that was not your question :)