What to do with very low offer on an item for sale

I join you as a "retired audio guy." Under the premise that you priced what you are selling realistically, based on comps, you have ZERO obligation to respond to an individual offering 25% of your asking price. Your offering a "good faith" additional 10% off to an individual making a reasonable offer would/could make sense. Remember, there are plenty of "bottom feeders" out there who feel entitled to offer whatever they want.

It's polite to respond.  You do not have to make a counter offer.  You could say something like -- it's priced like other recent sales and I think someone else will meet my number.  See what he says.  You'll be in better position to know how serious he is.

I get ridiculously absurd offers all the time, recently, $60k burmester 218 amp with warranty, asking $24k plus shipping, offered $16k

Ignore them. Simple

There will always be extreme low ballers. I would ignore them because none of them will pay what the item is actually worth!