Passive into Blackouts pre, blackouts pre and Fluence into switch.
Yeah, honestly I'd love to think I could get the two pickup families to work together properly rather than cordon them off from each other. I'd love to hear what the Fluence Modern bridge with Tone Zone S sound like together.
But what happens when two pre-amped signals meet each other at the switch? You're talking about wiring two preamps in parallel? Can you even do that without a mixer?
I think I'd have to run the Fluence into the Blackout Pre. Run them in series.
I feel like this is where it gets dicey, and it's what made me want to look into passive high-output bridge pickups in the first place. Ultimately I feel like that would be the path of least resistance.
The two preamps running in parallel don't affect each other - the Blackouts pre takes high-impedance passive pickup signals and converts them to low-impedance active signals (it does some other stuff too but for your purposes that's the part that matters). The mixer in this case is the switch - either pickup at 100%, or both in parallel (at a percentage corresponding to the output of each pickup). Wiring them in series would be problematic - in parallel they don't really interact.
Dicey, yes. It's an unconventional setup that most people don't go through the trouble of dealing with, which is why there's no plug-and-play or specialized hardware to make it easy, but it's not impossible or finicky by any standard - there's some hoops to jump through, but as long as you understand the problems you'll have, it's easy to work with.
The Bartolini Adjustable Gain Buffer
https://bartolini.net/product/agb/ is probably the easiest - it'll convert your signal to low-impedance and let you balance the output with an active pickup. It's also meant to be put inside the instrument, not as one of the controls.
The Blackouts Preamp is meant to take a full set of passive pickups and convert them to active pickups - it does some stuff tonally, bumps up the output, adds compression, and has a specific wiring scheme to increase noise cancellation with humbuckers. It
can be wired to do it, and I think might tonally make the pickups mesh a little better, but it's more of a hassle with mounting and stuff.