The OSMC version installed with the installer provided currently by HifiBerry was the September release. With that my HifiBerry Amp+ worked in combination with a Raspberry 2Pi. However, updating to a newer OSMC from that version did break it again.
With further searches I managed now to get it working on the version from February 2016. However the configuration has to be done manually by editing the config.txt: sudo nano /boot/config.txt and editing the two lines to:
dtoverlay=hifiberry-amp-overlay
dtparam=audio=off
The solution with defining dtparam=audio=off was discussed in the following threads:
I’m happy now it works. Still it would be good if the HifiBerry amp+ could be added as easily as the other audio boards.