[Ollo]

I use HifiberryDigi+ for DTS passtrough which works nicely as long as OMXplayer is disabled. However Dolby AC3 passthrough only works with OMXplayer enabled but this mess up the DTS passtrough. If OMXplay is enabled I only get a high frequent static noise (PCM32kHz according to my Sony receiver) when playing DTS files.

Not really understanding this. OMXPlayer doesn't support Hifiberry (or any ALSA device) at all.
If you enable HifiberryDigi in system/audio settings then OMXPlayer won't be used, so the OMXPlayer setting will have no effect.

Sorry I forget to mention that I use two optical inputs at the receiver. One from HifiberryDigi and one from my Philips TV.

These are the settings:
Audio output device: HDMI
Number of channels: 2
Enable passtrough: true
Passtrough output device: ALSA (Hifiberry)
Dolby Digital AC3 capable receiver: true

OMXPlayer disabled: DTS ok, AC3 not
OMXPlayer enabled: DTS static noise, AC3 OK