Simply explained this doofer contains 12 signal followers which are connected to each their own EQ. Signal followers are set to react to set frequencies in a way that it ducks the specific frequency while raising the frequencies one half note away in both directions.
Be careful while adjusting the 'Fill Size' and 'Fill Mass' as they can be very temperamental. If you leave the Limiter>Booster at the middle position it will limit the signal.
I planned to make it bigger and more advanced, but it was a lot of tedious work and it's already pretty CPU unfriendly, so this will have to do for now. I think it works pretty good though, once you got it fine tuned to the input it can add some warmth to the sound and change the character a bit and remove some rumble.
I'll probably make some more versions of this later with a wider range, but this version concentrates on the frequencies between 130Hz - 260Hz which is really more in the mid area. I'll add another version soon that includes the lower frequencies too.