I was able to successfully test the ADC Pro with an external USB DAC on one Pi. To do this, you must do the following:
– Install the latest kernel version (so that the ADC is supported)
– Deactivate the integrated audio in the raspberry settings
– Select the ADC in the Hifiberry plugin
– Select the USB DAC in the advanced options of Squeezelite (sysdefault)
– The ADC should be hw:0, the USB DAC hw:1, so you have to enter wavin:plughw:0 as a favourite
With these settings, the input to the ADC and the output to the USB DAC should work simultaneously.