Potentiometer (POTS) have different resistive curves: EG LOG, Antilog and Linear. That may explain the readings you are seeing. Try a LIN pot if you want an even change of resistance with rotation.
If you are looking to control volume at a low level (Pre Out – AMP in) and you don’t know the impedance of the circuits you are coupling then you could try a value around 10k. Speaker level pots used for low cost “crude” fading or level balance are usually wire-wound and between 4 to 30 ohms.