The 100K stepped attenuator (Level Control, Volume Knob) is in parallel with a 10K (Phase Linear) or 39K (White Oak Audio) pull down resistors. Using Ohms Law, we can see the input resistance (impedance is frequency driven) of the PL400 is 9.999 Ohms (Phase Linear) or 28K (White Oak Audio).
In my case the 100K stepped attenuator was paired with a White Oak Audio Control Board. I would not recommend a 100K attenuator for use in a PL400 with a Phase Linear control board. The input resistance would be lower than 10K, and 10K is already too low,