IMO the best way to test the difference between gain and master is to hook up a passive instrument directly volume and tone at max with no pedals that could have a buffer circuit. (passive P basses work for this quite well)
At the same SPL levels You'd hear that cranking up gain with low master is a brighter sound vs. cranking up master and lowering gain.
Reason is that Your instrument's pickup(inductor)+passive tone capacitor+cable(which also has some capacitance, etc.) and the gain knob's resistance becomes a passive
RLC circuit. If You add more resistance, You lose (mostly)high frequencies. (it's a bit more complicated, but this is the fundamentals

)
(the pickup's internal resistance and the passive tone circuit in the bass also play here, I try to simplify this just for the sake of making an example)
This input signal drifference also effects vintage(compression) and OD. And to a degree how the tone stack in the preamp behaves.
If I'm using my rumble only as a clean amp, I usually crank gain more for a more "lively" "brighter" tone. Then adjust highs with the EQ and/or on the instrument.
If I'm also using rumble's OD (almost always Drive at minimum) with an external switch pedal, I tend to lower gain (and crank master) for a more "vintage" overdrive sound, because higher signal distorts more and becomes too raspy compared to clean without adjusting tone on my bass, which is inconvinient mid-song.
(to my tase! I like my driven tone to have just a little breakup. as it removes a tad bit of bass, it also effectively becomes a mid boost, which is my goal, as I try to keep bass levels somewhat equal between clean and OD)
HTH. YMMV.