If you are using the analog RCA output from your CD player then the CD player’s DAC has already done the Digital to Analogue Conversion (hence the acronym D.A.C.) so the WiiM then has to convert that back to digital via its ADC before either passing that to your amp over its digital outputs for the amp’s DAC to convert it back to analog, or use the WiiM’s DAC to convert it back to analogue if you’re using the WiiM’s analogue outputs into your amp.
The cleanest solution is to use your CD’s digital optical output into the WiiM so the WiiM’s more modern DAC can do the conversion and convert the signal to analogue to feed your amp.