Single-band parametric equalizer, which boosts "a select band of frequencies, so that instruments with a tendency to be 'muddy'-such as electric bass-could be brought out in the mix". The spectrum knob sets the frequency range in the interval 500Hz to 5kHz. The balance knob sets the peak within the range.
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Visible in a video of Billy's huge collection of guitar pedals called Stompland, although he doesn't pick it up or mention it by name.