Every engineer learns it early: boost the lows on a passive EQ, then cut just above the boost. The kick gets heavier and the mix gets cleaner in the same gesture. Here is why it works, where it breaks, and what happens when the dip is allowed to move.
What the trick actually does
On a Pultec-style equaliser, the low boost and the low cut share one frequency selector, but their curves are not mirror images. The boost is broad; the attenuation is narrower and sits slightly higher. Turn both up and the sum is a bump below the selected frequency followed by a dip just above it: more weight around 60 Hz, less mud between 150 and 250 Hz.
That shape is the whole point. Low-end problems in a mix rarely come from a lack of bass. They come from the octave above it, where the body of the kick, the bass, the low end of the guitars and the room all pile up. The trick adds the weight you want and, in the same move, clears the region that made the weight sound muddy.
Where it breaks
The dip sits at a frequency you chose. The material does not know that. A kick tuned to F sits at 44 Hz on one song and 55 Hz on the next; a bass line moves through the very octave the dip is parked in; a sampled 808 slides. The bump and the dip stay where you put them, so on a good day the dip lands on the mud, and on a bad day it lands on the fundamental you were trying to keep.
The usual fix is to automate the frequency, or to split the song into sections with different settings. Both work. Both are also a way of doing by hand what the equaliser could be doing by itself: following the material.
The Pultec trick without the fixed frequency
Spectral Mass Rebalance does not think in frequencies. You draw a band; it measures where the energy inside that band actually sits, then tilts the balance around that point. The total energy of the band stays constant. What the top of the band loses, the bottom gains, and nothing is added or removed.
To turn that into a low-end trick, you draw a single band over the low end, from 10 to 300 Hz, and tilt it toward the lows. The bottom of the band comes up, the top of the band goes down: that is the bump and the dip, in one control. The upper edge of the band is not a hard cut but a slope spread over an octave, so the effect fades into the rest of the mix rather than stopping at 300 Hz. And because the pivot sits on the centre of mass of the kit, the dip follows the kick when it moves. A touch of envelope follower, 55 % here, lets the tilt breathe with each hit instead of sitting still.
Below is that setting on a drum loop. The interface is the plugin itself, rendered offline, in sync with the sound. BYPASS takes the plugin out of the sound and the picture at the same instant. DELTA plays only what changed: on this loop it sits about 21 dB under the signal, roughly what a well-set passive EQ would move.