Warning!!! This will void your warranty.

This hack replaces the internal gain potentiometer with a case mounted potentiometer that is adjustable outside the case.

I used these (https://www.amazon.com/gp/product/B01LZ74KU3) potentiometers mainly because they came with matching knobs. I cut 2mm off the end of the shaft with a Dremel cutter so the knob would be almost flush with the case. You could also use a hacksaw.