An addendum: one of the pair now persists in finding a "mains fault", after behaving very nicely for 2 years. Changing to a different circuit? Same problem.
So I see that the nearest authorized repair can be gotten in Austin,Texas or Syracuse, NY or NYC. A shop in LA professes to be authorized, but all of these are hundreds of miles away. I have written to dealers and others who do electronic service within reach, and so far, nothing.
I guess I could grit my teeth and send the amp away, but therein lies another problem: I bought 'em both used, and neither came with "official" shipping cartons. Levinson once was willing to sell such a carton for <gasp> ~$600, but no longer.
So the best course now appears to have a crate built (the thing weighs 85#), to buy robust cardboard cartons and a ton of packing peanuts ... or to write it off and buy another. <grumble, grumble>