Every engineer who has used a passive program equalizer learns the same trick in the first week: turn up the low boost and the low attenuation at the same frequency, at the same time. On paper it should do nothing. In practice it produces a lift in the bass with a dip just above it, and that curve tightens a kick drum or a bass guitar in a way no single band of a parametric EQ quite manages. Here is why it works, and how to use it.
Why a passive network does not cancel
A passive equalizer has no gain of its own. Its filter network can only take away, and the tube amplifier after it makes the loss back. The boost and attenuation controls on the low band are not two opposite gain stages; they are two different filters in the same passive network, and they do not have the same shape.
The boost is a shelf that lifts from the selected frequency downward. The attenuation is a shelf too, but its corner sits higher than the boost's for the same frequency setting, and its slope is different. Add them together and the frequencies below the corner come out roughly where they started, while the frequencies just above the corner, where the cut is already working but the boost has not caught up, drop. The result is a lift below and a dip above: a resonant bump with a notch on its shoulder.
Push Pull reproduces the passive network, the push-pull tube amplifier and the output transformer, so the trick works the way it does on hardware. The low band selects 20, 30, 60 or 100 cps for both BOOST (a shelf lift to +16 dB) and ATTEN (a shelf cut to −20 dB), and because the filter is passive, running both at once gives the lift-and-dip rather than silence.
Using it on a kick
Set LOW FREQUENCY to 60. Bring BOOST up until the kick has the weight you want, then bring ATTEN up behind it. As the attenuation comes in, the mud around 150 to 250 Hz drops away while the thump underneath stays. The kick gets bigger and cleaner at the same time, which is the opposite of what a broad boost does on its own.
On a bass guitar, 30 or 60 tends to work depending on the tuning; on a full mix, 20 or 30 with small amounts of each. The frequency levers click between the fixed positions but STEPLESS lets them sweep continuously if the right spot is between two settings.
The high band and the tube stage
The high band works differently. HIGH FREQUENCY picks a resonant peak from 3 to 16 kc with BOOST to +20 dB, and BANDWIDTH runs the peak from SHARP to BROAD. There is a separate high shelf cut, ATTEN SEL, at 5, 10 or 20 kc. The same principle applies: a broad boost at 10 kc with a shelf cut at 20 kc adds presence without adding hiss, because the cut takes the very top off the boost.
IRON drives the push-pull tube stage harder with the level compensated, so what you hear is the harmonics changing rather than the volume rising. The flux meter shows the output transformer working. On a mix bus a little IRON is often the difference between an EQ that sounds like a plugin and one that sounds like a piece of equipment.
Working modes
Push Pull runs in STEREO, DUAL or M/S, with COPY to match channels. M/S is worth trying for the low-end trick: apply the lift-and-dip to the middle only and the sides stay untouched, which keeps a wide mix wide while the centre tightens. Auto Gain keeps comparisons loudness-matched, MIX blends the equalized signal with the dry one for parallel use, VINTAGE applies per-instance component tolerances so two instances are not identical, and NOISE adds the floor of the original if you want it. Oversampling runs from 1× to 8×.
The lift-and-dip is a thing you have to hear to trust. Put Push Pull on a kick, set 60, and bring BOOST and ATTEN up together; the 14-day free trial is the full version, so that experiment costs nothing.