Volume pedal that retains high frequencies when reducing volume- going crazy

Based on recommendations from this site I have tried two different FV-50L (low impedance) pedals as well as a similar low impedance Bespeco VM14L. Every one of them causes the high frequencies to drop off significantly when reducing volume.

Details on how I’m using it: BB connected in stereo to the volume pedal (2 cables) and then out to PA (Behringer X32). I only need to be able to move from “fast song volume” versus “slow song volume” rather than any complete fadeouts. So I’m using the “minimum volume” control on the volume pedal so that at lowest setting, the volume reduction is in the order of only about c. 6db. Yet hihats and cymbals basically disappear as a result of the high frequency loss.

@BrennanSingularSound previously posted that output impedance of the BB is 26 ohms, so well below anything that should lead to an impedance mismatch.

What am I doing wrong?