Is a offer of -20 percent on an OBO ad insulting?

Anyone who is insulted by an offer that is only -20% should ask rock bottom and list as "firm". If you list obo you have to leave some room for negotiation. I have responded with "too low" and I have ignored stupid low offers. Dont think its rude if the offer is obviously ridiculous. Dont take it seriously, feces transpires.

Seller could have added "lowballers will be cheerfully ignored" or "price is firm" or similar text as I have seen many times here on Audiogon.

As a newbie to the 'sport', I have found it isn't always easy knowing what etiquette is in this arena. I for one probably wouldn't know what is fair game for a used Krell, and may have incurred the wrath of this seller too.

Anybody with an OBO ad calling a -20% offer ridiculous is probably out of this world. If he thinks his gear commands a PARTICULAR price then he has to state it as PRICE IS FIRM, not OBO.
Let's not waste time with that sort of sellers; there are lots of gears on sale by others..

how long does it take to type n o and click send message button?

If his ad included OBO then you should be able to offer any reasonble amount. Otherwise, he shouldn't have listed the ad that way. Anyway, he sounds like your typical audio snob, probably a salesman in a high-end audio store.

Besides, Krell is way over priced for what is essentially a big power supply.

Jim