Unfortunately I can't establish a link to my Aragon D2A2 with the Digi+ pro. I do not have another DAC to test with and as usual the error could lie where ever...
Raspian Jessie, Linux 4.4.38, BRANCH=next on RPi2 with Digi+ pro
adi@hipi:~ $ dmesg | grep erry
[ 0.000000] Machine model: Raspberry Pi 2 Model B Rev 1.1
[ 0.722633] raspberrypi-firmware soc:firmware: Attached to firmware from 2016-12-09 15:12
[ 5.444493] snd-hifiberry-digi soc:sound: ASoC: CODEC DAI wm8804-spdif not registered
[ 5.444603] snd-hifiberry-digi soc:sound: snd_soc_register_card() failed: -517
[ 5.535171] snd-hifiberry-digi soc:sound: wm8804-spdif <-> 3f203000.i2s mapping ok
adi@hipi:~ $ aplay -l
**** 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
The pi is connected by coax to the DAC, the output on alsamixer is set to S/PDIF (not AIF).
If I start
play -r 44100 -c 2 -b 16 -n synth sine 500
the program runs, but there is no red light going on the board nor is the "lock" light of the D2A2 turning on.
Using other inputs for the D2A2 works.
The hifiberry overlay is detected automatically, standard sound overlay is commented out, /etc/asound.conf is as suggested by the support docs.
Do I have an error in my setup or is my DAC too old? Could some one please help me to verify my setup?