Why "must" electronics withstand 600V. There is no standard. People in repair newsgroups say some electronics has protection and some does not. That is also what I have seen.
Protection that is often used is a MOV from line to neutral (and sometimes MOVs to ground). But according to w that can not possibly work - there is no less than 10 ft connection to an earthing electrode.
Wow - you used "if".
What we don't know: Is OP in a high lightning area? Lightning around the time of failures? Other major surge sources? What protection was provided by the manufacturers? What is the electrical system earthing? Is there a neutral-ground bond at the service? Loose neutral? Schlocky wiring? Coincidence?
I give up. What do you think happened in the 1990 NEC?