First, welcome to the Shack!
Are you saying that you have a 3 dB hole at around 50 Hz, and if you boost it up you'll have flat response down to 15 Hz? Or are you needing a shelving filter as brucek noted that will boost everything from 50 Hz down?
The BFD can do the former with no problem, but we can't recommend anything for you since you didn't post a response graph. How much to boost the filter, and at what frequency - it looks like you have a handle on that. The only remaining variable would be the bandwidth - i.e., how wide or narrow the filter would need to be to "fill the hole," as it were.
If you can't post a graph, or don't want to you can find a formula for calculating parametric EQ filters on pp. 9-11 of this subwoofer manual.