I’ve never tried it, but some of the voodoo labs power supplies have a knob you use to simulate a dying battery.
related to this, but totally off topic otherwise (apologies):I have an old boss hm-2 that takes an oddball boss 12V adapter. It's fairly fuzzy in its normal state, but it gets super splatty almost sort of gated fuzzy awesome when you underpower it with a 'standard' 9V adapter