The digital audio part of the Digi+ Pro can be powered independent from the power supply of the Raspberry Pi. However, a small modification on the board is needed:
You need to de-solder the resistor R3 and connect your external 3.3V power supply to the pin header P5.
There is another 5V header on the left. This is the power supply for the Toslink output module and the Raspberry Pi.
Note that this modification voids the warranty. We recommend it only for experienced makers.