I got a little more info, though not the power amp. Per advice from
Sweetwater, they want to run the passive sub daisy chained with the mains. If I understand correctly it goes
Mixer Left Out -> Power Chan 1 -> main1 -> main2 right now with us not having the sub.
Mixer Mon Out -> Power Chan 2 -> mon1 - mon2 - mon3 - etc.
I don't think putting a passive sub in the chain is right at all because you would never be able to dial in the volume correctly...
Taking your advice I think mixer Right out can go to a powered sub that has it's own crossover, like a Peavy 18D. Am I getting closer?