Question I have difficulties to set the right output volume when it is around the 9 o'clock position. The control is very sensitive, and the volume level  increases a lot even if I turn very slightly the knob. Is it possible to recalibrate the master volume pot   Answer Indeed, the sensitivity is quite logarithmic at this specific point. If you play often with the level set in this area, you can use the preset level as a work around:
  • Set the master knob  in a higher position - i.e. 10 o'clock. At this point your level might be too loud.   
  • Press shift and use the "tubetone"/ Preset level to adjust your volume.
If this is a level you often use (i.e. rehearsal) , save it as a preset.