Hey Billy and Barry, thank you for getting back to me.
The pots have rubber stickers on the back, they dont have contact with any solder leads. I measured a bit today and found out that there indeed is a 9V (vs. 9.4 V directly on the 9V – Gnd) reading between Out and Ground. I removed the Mosfets and the IC but the voltage remains. Tomorrow I will reseat C9 and C10 and also check all the solder points of the power cuircuit to see if there is a short with a neighboring pin.
@Billy: You instantly mentioned the caps. Would a broken cap cause this behaviour too or just a faulty soldered one? And also are you referring to only the ones in the power circuit or also C1 to C8?
@Barry: This is just a reflection of R7. It seems this Mosfet is literally new and shiny. 😉
Thanks again for your help.