Or you could consider spilling a few more bucks for a decent crossover. I use a coustic XM-3 but due to ground loo problems and therefore whining caused by this poor old beaten unit I only use it as a Low Pass Filter for the sub.
I run the front and rear speakers off a Y RCA cable from the front and use the amp's gain to adjust the listening position (Front - rear balance)
The EQ on the Empeg is set to be a HPF on the front and a LPF on the rear. This LPF is duplicated somewhat in the crossover.

Works well for me

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Murray