MC275 KT88 tubes blowing left and right

What do you actually know about the tubes? While I assume they were 'matched' that is often an empty or less than useful promise.

Some tubes require less current to bias properly, some more. Auto bias circuits usually operate in a pre-set range and it is possible to put in a tube which exceeds that range. That's why I like to have amps which allow for manual bias adjustment as opposed to 'auto bias' circuits. If I get a 'hot' tube I can easily identify it when I bias the tubes. In an auto bias circuit some times I only find out when the tube turns red or blows out.

FWIW a lot of manufacturers take the tubes to their limits to optimize the spec's so the margin can be small when you go from the design spec's to biasing it too hot. You really can't rely on the 'matching' process used by manufacturers or distributors/dealers that you don't have to pay extra for.

FWIW my guess is that the problem is in tube selection and biasing. It is possible for an amp to run on one tube per channel, it just won't operate in a push/pull configuration.

Another consideration - often when you have tube's arcing they can take out resistors in the bias circuit - and the amps still runs after you replace in main fuse. So it is best to have your amp checked out by a techie to make sure everything is in order. He might even source the problem of your tubes blowing out.

The tubes are all new. Not even more than 25 hours for each of them.

First off, power strips can really mess up the sound, especially if they have cheap surge suppressors in them, i.e. MOV's. I would avoid them. That, however, has nothing to do with your tubes arcing. Sometimes because of the circuit topology, pairs of output tubes fail. Depending on the way the tube arced, increased current draw occurs in the tubes mate and that tube fails as well. Not at all uncommon. How many hours do you have on your output tubes? That may determine if the whole set should be replaced. Since all the output tubes were installed at the same time, they all have the same amount of hours on them. Replacing output tubes in ones and twos is generally not a good idea.