Save the DSP settings you’ve made for mono to a > Preset – you could call it
“1 - Mono”;
afaik Presets include all settings, not just the filters, so headroom adjustment would be included.
Save the standard DSP settings you normally use to another > Preset – you could call it
“0 - Default”.
Now you can easily switch between those two saved presets. It’s almost like a Mono button.