Here's my Sadowsky preamp epiphany:
The blend knob is the mids knob. I feel like this also makes the reverse wiring of the blend knob make sense - rotate clockwise for more mids, anticlockwise for less mids. Bass and treble knob work the same, clockwise for more and anticlockwise for less. For me, the pickup blend decides how much I want to stand out.
I add bass to taste. I like to get some booty, so I usually add some bass. The treble knob is for when my strings are dead or I want to do some slapping. The treble knob helps get that good scoopy sound when the blend is centered and makes me feel like I can pull off Marcus Miller licks lol.
Passive tone knob? Not useful. Don't care for it. I crank it open and ignore it.