Question

tops are powered and subs are passive

Answer 1

You can run your channels into a AUX, setup the EQ on the AUX master to a x/over point you want and then send that aux channel to the Sub amp.

Answer 2

I don't think you can aux fed subs with the Driverack PA2 at least no easily because you can't customize the output channels in the PA2, the DriveRack VENU360 would make it a lot easier to achieve that.

The only workaround I can think of is if you're running the whole PA in mono:  

24R Main Left output to "PA2 Left input" to "Left High output" for all the top speakers.

24R Sub Aux ouput to "PA2 Right input" to "Right Low output" for the sub amp.