On paper it looks like a reasonable difference. So you have the grid stopper resitor, basically the input resistor to the first stage. With Bright off it's 47k straight in, with Bright on it goes to 100k with a 4.7nF capacitor in parallel. But apparently that just isn't an audible difference?
I wonder if it might be more noticeable in a DI or full-range PA setting vs thru a standard bass cabinet, and also if the bass in question is, say, a thumpy P with antediluvian-aged flats vs a sprightly active bass with Nordy Big Singles and fresh rounds....
), I'm also looking to abuse the gain heavily, and would take that over extra headroom.