I’ve been running a sealed Ultimax with an inuke DSP amplifier for a couple years now. Only recently did I learn that there are ways around the limitations on the DSP filters going only down to 20 Hz. Instead of using a low shelf filter, a negative high shelf will give lower extension to the filter. You probably have a couple questions.
Q: Won’t a negative filter reduce my output?
A: Yes, but you can compensate with increased gain
Q: How does a negative high shelf go lower in frequency than a low shelf filter?
A: It all comes down to which side of the filter the 20Hz knee is at. A 20Hz +6db low shelf and 20Hz -6db high shelf filter will have the same curve shape but at different frequencies. The peak of the low shelf will be at 20Hz, meaning the rise begins at a higher frequency than 20Hz until it levels out at 20Hz. The bottom of the negative high shelf filter will be at 20Hz, meaning that the overall response rises at 20Hz through lower frequencies. The strength of the filter determines how low in frequency the filter peaks.