It turned out that a 20K ALPS motor potentiometer is not that easy to get. So best way forward is to implement a new volume control with the following requirements:

1. The volume control has to support two separated channels in balanced mode
2. It has to fit into the existing housing of course
3. If a supply voltage is required, it shall work with an existing voltage given by the pre
4. Current shall not exceed ~300mA/5V
   
5. The given design uses a ALPS rotary encoder which signals are translated into two control voltages for the potentiometers. Hence either the control voltage or the encoder signals shall be used for setting the volume.
6. To fit into the circuit flawlessly, volume control shall remain resistor based, digital volume controls like the BurrBrown PGA2310 introduce special requirements for input and output resistances.