It's possible to 'cheat' the EQ a little to do what you want to do. As an example, I could create a fairly broad (Q=0.5) boost at 20Hz, and then a couple of narrower (Q=1.0-2.0) cuts at say, 30Hz and 60Hz or so. The three filters will interact, and the result would be that the boosted area above 20Hz would be nulled out by the cuts, but the area below 20Hz would not -- hence a sub-20Hz boost. I'm doing this in my head right now, so the exact values for Q and Fc might be off, but the principle is sound. The only issue is whether the DCX rolls off quickly below 15Hz or 10Hz, in which case one might be 'spitting into the wind,' so to speak.
Here is an example from EasyQ. It doesn't go down below 20Hz, either, so I constructed the example to boost 40Hz by a Q=0.5 boost at 50Hz, then cuts at 75Hz, 125Hz, and 200Hz. If we pretend that the issue is a lack of boost below 50Hz, the example makes sense.
On of the issues with doing this is that there are smaller distortions, like the dip at 70Hz in the picture. If I took an hour or so and played with the Q and Fc values, I could probably improve the response. Adding more narrow cut filters might help, too: I've simulated 'brick wall' filters with 5-6 filters in the past.