I am using a Digi+ Pro with my Raspberry Pi 3. I have tried Volumio and Raspbian and get the same results. The audio is output to my Audio Receiver, but the sounds only plays when the volume is very high. Even then it's quiet a distorted. The output of aplay -l is:
**** List of PLAYBACK Hardware Devices ****
card 0: sndrpihifiberry [snd_rpi_hifiberry_digi], device 0: HiFiBerry Digi+ Pro HiFi wm8804-spdif-0 
Subdevice #0: subdevice #0
Any advice you can provide will be appreciated.