Subs with onboard crossover are usually powered as the crossover is done at line level not speaker level.
Signal from mixer goes to sub which amps the lows and sends on the highs for whatever is amplifying those.
If passive sub you need amp for it and a rack crossover.
As currently constituted our PA is a powered mixer driving 2 passive mains. We also use the mixer line out to drive 2 powered monitors.
How would we connect the sub in this configuration?