Thank you for the detailed post. You can also write in German if that’s more convenient for you 😉
Have you tried the HiFiBerry Plugin yet? Please set „none“ in Raspberry Settings sound card selection and save and reboot.
Then use the HiFiBerry Plugin to select the AMP2 and reboot.
Check the debug info in Audioplayer to see if the hifiberry card was recognized.
The AMP2 does not have an activity LED.