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.
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