From the Mackie manual “1. Channel GAIN
The GAIN controls adjust the input sensitivity of the
mic and line inputs on channels 1-2. This allows the
signal from the outside world to be adjusted to optimal
internal operating levels.
If a mic-level signal is plugged into the XLR combo
jack, there is 0 dB of gain (unity gain) with the knob
turned all the way down, ramping up to 60 dB of gain
fully up.
When using the balanced line input of the combo jack
(1/4” TRS connector), there is 20 dB of attenuation
all the way down, and 40 dB of gain fully up, with a