Are you familiar with the NAD C 390DD? It amplifies digital signals from, for example, the digital output of a CD transport or player (converting those signals into the analog domain at the speaker outputs). The C 390DD does not even accept analog signals without adding a daughter card purchased separately.
The backplane of the C 390DD has, I think, two coaxial digital inputs (RCA connectors) and one AES/EBU digital input (XLR connector).
I'm asking whether anyone has compared the sound quality of the amplifier via these two digital inputs, preferably from the same digital source (i.e., one with both RCA and XLR digital outputs).