"And the Geek shall inherit the Earth" !
Dave.
Hey now! Both of my bands usually run direct into my rack machine running Reaper for amp sims and mixing. I plug two (redundant) line-level XLR cables into the stage snake and tell the sound guy "turn it up and blame me."
Each of the live sources in those bands is either a guitar pedal or a keyboard, and they each plug a TRS>XLR cable into the mic pres on my US1641. There is one of my dad's keyboards that buzzes bad when we do that (a Moog something that we think is just broken, but I'm not allowed to try to fix) so I swap in a DI, but I've used a pretty diverse set of devices this way with no other real problems.
Any soundguy with a clue will just pad or trim as necessary at the board. I always make a point to warn them that it's going to be a kind of hot line level, they say "OK" and move on. A passive DI is pretty idiot proof. It really just works and nobody has to think about it, but straight wire should work most of the time and when it does, it's usually better.
OP - I think the BBE is only going to put out so much level anyway. Whether the boost is before it or in the loop, it works out the same. As long as your guitar level with the boost on isn't more than the other thing can reproduce, you should be fine. There's plenty of knobs between all of these things to make that happen. Whether that's ideal gain staging-wise...well it's probably not...but live work can be pretty forgiving of a little noise. Just make sure youre not getting unwanted distortion.
But really, I'm ass-u-me-ing that this is an acoustic-electric you're plugging into the thing? That could go straight to the FOH and might be even older than the BBE on its own. The booster pedal, if it's a 9V stompbox, could potentially put out most of that 9V p2p depending on its design, but probably more like 4-6.
Actually the whole thing kind of terrifies me from a soundguy's perspective. I just see sudden, massive changes in the level you're sending me of which I have no warning and no control... I guess that's kind of the opposite of how I started this post, but YOU don't even know which hole to stick it in, and so I don't trust you.