There is a couple ways to do it. I plan to add a sub to a triamp system and in my case the low pass from the DEQX will go to a Behringer DCX to split the woofers from the sub. Another DEQX user used Y-connectors on the low pass outputs so that he could run one set of cables to his woofer amp and another set to his DCX (your unit would work too) that crossed and eq'd his sub.
Certainly cheaper than buying another DEQX. I am not sure FIR vs. IIR will matter in the bass region, so no need really to go with the super solution. It will complicate the setup a bit though. You are going to need to figure out the phase and time offset for the woofer and subs indepently of setting up the DEQX xo's, because to the DEQX the lower section will look as just one pair of woofers and it will treat it as such.
I am in on the DCX group buy on diyaudio.com for this reason. I plan to add my Avalanche 15" in a sealed box for helping out below ~50hz. That will take some stress off the dipole woofers.
