Hi Mathias,
Since the chorus does not have an effect on your issue, chances are that the source of the problem is after the chorus.
If you look at the overall diagram, you'll notice that the audio signal leaving the chorus goes through two circuits, both located on the PRA board, before reaching the output.
The first is dedicated to the EXP effect and the second to the preamplifier. There are 5 variable resistors located there VR11, VR12, VR6, VR7, and VR8. You could check them.
Have a look at page 29 of the adjustment procedure guide.
Joachim
