John - this is pretty easy to do. I am assuming you are aux driving your subs with a separate feed such as Subgroup fader or perhaps panning all your bass instruments Left and other channels Right. As you have already discovered you must run everything mono since there are only two inputs on the DRPA.
Let's say you are running your mono aux sub signal into the DRPA Left input. Then you run your other instruments/vocals mono output into the DRPA Right input. Your DRPA output connections are as follows:
Left LOW out > Subs
Right Mid out > Full range speakers A
Right High out > Full range speakers B
You must get these outputs correct both in terms of L/R and Low/Mid/High.
Then to the DRPA software setup. One easy way is to start with a 2X6 configuration. That is Left and Right inputs going to Left/Right Low/Mid/High. You can build one from scratch or just find a preset. Figure that you will likely be changing all the other settings such as xover, geq, etc. anyway.
Here is an example program showing a 2x6 configuration. I just renamed one of the presets to
AUXSUB+ 2x6.
The key item to set up is the crossover. So press the Xover button and press the NEXT PAGE button (if needed )enough times to get to the first screen for the Lows . Then set your subs high pass filter as needed. This is where you would set the gain for the subs also. These numbers are for my system but will be different for yours. Note that these traces on the screen will look something like this when you are done, but not necessairly when you start. Don't let that confuse you.
Press the Next button to get to the next screen for lows. Then set the low pass (crossover point) and filter type for the subs. In my case I used 100Hz and an LR24 filter type.
Now for the MID channel press Next again. The default will probably have the mids being a bandpass something like 100Hz to 4kHz. The idea is to change the Low Pass on the Mids so that it goes all the way up to OUT. First set the high pass (crossover point). In my case I wanted 100Hz and needed a gain of -5dB for good acoustic balance with the subs. Yours will likely be different.
Press Next and change the Low Pass for the mids to be all the way up to OUT. Now that mid channel will output full range from 100Hz to 20kHz
Finally press Next page to setup the High outs for full range. The default low pass for the High channel is all the way up to OUT and is non-adjustable. That is OK and what we want. The thing you want to change is the High Pass on this channel. Take it down to where you want the crossover to be. In my case, since these were different speakers, they sounded better with a 90Hz crossover point and I needed a different gain as well.
You still need to review all the other items like GEQ, PEQ, etc. to make sure they are set up the way you want. Hope this helps.