mr dave

you could try using a volume pedal with a built in limiter. it's basically just a little knob on the side that controls the range of the pedal. you can get cheap plastic ones for about $40-50.

set the volume on the mixer to be at your max desired volume. then set the limiter so that when the pedal is 'down' the volume is at your regular level. at that point you can use the pedal as a fader to smoothly go between your two desired volume ranges.

a direct a/b stompbox might work but it would be a harsher transition between volumes.

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