bx_subfilter

bx_subfilter, a freeware utility for adding clarity and definition to the low-end of your mix.

The plugin is based on the common concept of using a resonant high-pass filter to clean up the extreme low end and boost a target frequency range at the same time. Essentially, the filter tames the content below the cutoff frequency and adds clarity and definition at the cutoff point. It’s a pretty neat technique and certainly a good alternative to using a combination of a non-resonant HP filter and peak filter to do the job.

bx_subfilter was developed with this specific technique in mind and I’m pretty sure that Brainworx designed its curve and behavior to translate well to processing the sub-bass frequency range. Apart from the cutoff knob (which is labeled Tight Punch on the user interface) bx_subfilter also features a fixed band-pass filter control which boosts the low end, a resonance control for adjusting the amount of volume boost at the cutoff point, and input/output gain controls.

https://www.plugin-alliance.com/en/products/bx_subfilter.html