Enable pass-through and select the right card.
With your OS configured move to System -> System settings -> Audio
Verify that the card is snd_rpi_justboom_digi,S/PDIF
then enable “Allow passthrough”
and finally select snd_rpi_justboom_digi,S/PDIF in “Passthrough output device”
In general the surround system to which you are connecting your JustBoom to will be able at this point to detect that the incoming signal is a valid DTS / Dolby / surround sound signal.
You might want to download a test sample to verify your settings. Here are some links that provide free to download samples: