Thanks for the answer. Just removed the adapter and the ZERO worked - although sound was a little bit muffled (otherwise the opamp wouldn't be necessary). But  no destortion and full amplification. Just ordered a new OPA627. Let's see...

I messed a little bit around with my Tianyun Zero because the OPA627 seemed a little bit harsh on my K701. Unfortunately i plugged the OPA adapter the wrong way back to the base plate (180 degrees switched - it was pretty dark in my room - no excuse anyway). Now sound is destorted and almost not...