I use the DBX DriveRack PA2 to control my subs. In my opinion you are doing it wrong. Take one output from your preamp and send it full range to your amp. Take the second output from the preamp and send that to the DBX and use the software on that to set the crossover and levels etc. etc.. Run the subs with the crossover set to the maximum and set the level to approximate what you want but fine tune with the DBX. The absolute last thing you want to do is have the DBX in between your preamp and your power amp. The cheap DA/AD converters in it will destroy the quality of your sound but it is completely irrelevant for the subs. If you don't have two outputs from your preamp I suppose you could always get a y-splitter.
One DAC to Rule it All
Sorry for the "click-bait" headline. The digital side of my system is an Auralic G1 sending to an OKTO DAC8 Stereo. I like the sound a lot, but I also use a DBX Venu360 digital crossover system to split the signal to my mains and subs. The DBX gets an analog signal from my preamp (Benchmark HP4), but to do its function the DBX unit converts to digital, then back to analog at 96Khz. My question is, since the DBX unit has the capability to receive a digital signal (AES/EBU), would it be better to use just one DAC (DBX) to do it all, or stick with the double converting and keep the OKTO in the system?