I have a pre-amp harness in a Fender Jazz Deluxe bass that I might feel inclined to change. I'm not sure yet, but if I do the Fender Pre-amp could be available.
What it has is
Vol,
Blend,
Stacked Treble/Bass,
Mids
So there are 4 knobs; 1 more than you have. IF we can come to an agreement, and if it's the right thing for you, I could sell you this. I'm in no rush though. Let's see if anyone else comes up with any more info.
There's no passive mode, but maybe you can fit a by-pass switch after the blend and by-passing the tone pots/pre-amp, linking the blend straight to the jack.
What does the seller have to say on the subject?