If you are talking about the BFD DSP1124, it is a poor choice to use for subsonic filtering. The lowest "center" frequency you can set is 20Hz on that unit, and it order to effectively lower the content below 20Hz you have to set a rather large Q value which is going to affect content above 20Hz. Then to raise the frequencies back up that you didn't want lowered, you have to set another parametric filter to boost those.
Not saying it can't be done, but it would be better to just get a unit that supports a normal high-pass filter.