First off, I would strongly suggest running the whole rig in mono, not stereo.
*Keep the mixer's pans centered. Use the (L) mains output to feed the crossover. This will require a single XLR cable.
*Configure the crossover for "stereo 2 way". Yeah, I know we're running this in mono but we'll only be running the crossover's left channel so it won't really be stereo. Plug the XLR cable in the xover's left input.
*Run an XLR cable from the xover's (L) high output to one of your two powered main speakers. Daisy chain to the second with another XLR cable. Now, the subs....
*Plug an XLR cable into the xover's (L) low output. Set the LF (low frequency) button to "summed". Run the cable to whatever power amp is fueling your subwoofer enclosures.
Set your stereo 2-way crossover freq @ 100 hz for starter. Sidenote: this is a dbx knock-off and the front panel is confusing as it offers full parameter control of 2-way (highs/lows), 3-way (highs/mids/lows), and 4-way mono (highs/high mids/low mids/lows). If you're having trouble figuring out what does what, let me know.
"...my whole body's a weapon" - Luther Heggs