A passive crossover requires a specific load to work with. Too large resistance and it is underdamped, making a peak near the crossover frequency. Too little resistance and it is overdamped, making a shallower slope than expected. In either case, the crossover frequency changes. And since the woofer isn't resistive but rather reactive, other strange interactions occur. Most people add a Zobel filter to mitigate this, but that requires a large power resistor. I use a 100 watt resistor for Zobels on the woofers in some loudspeakers with passive crossovers, for example.Why not run an active crossover instead? For subs, I think it's a better option.
You could even do something like this, with a 12V wall wart supply:
It's cheap and configurable. It will work just fine for you, I think. But it will require separate amplifiers for subs and mains.
If you don't already have an amplifier for the subs, you can either purchase a separate discrete amplifier or you might want to use a plate amp instead. It has the sub crossover built-in, so you won't need the crossover above if you go this route.