Brad,
The CEX changes things a bit. I haven't worked with that exact unit, so I'll try and stay out of trouble by being general, and hope others will correct me if I'm wrong.
1. The 4215's are -3dB @50 Hz, and -10dB @ 36Hz. So send your signal to the CEX, set one set of outputs to HP filter @ 36-40Hz and feed this to the 4215 amp(s). The box is tuned to 48Hz, so you may have excursion/distortion issues, at high power, below 48Hz. Only you can answer this.
2. Set another set of outputs to LP around 80-100Hz (to taste) and run this to your sub amp(s).
With the main EQ flat, adjust the output of the two sections to balance the subs and mains. Then, add your EQ to correct or enhance.
Any other opinions?
daver