One quick guess would be a KT88 may have shorted from getting moved, or just its time.
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No one said all four have gone bad at once. You need to test all four to see if any of them are bad. In most cases, that's the easiest, and cheapest method. If all the tubes are good, then get the amp checked for a bias problem.
Atmasphere, if one tube has a short, couldn't it pull the minus bias voltage for the other three down with it? Also, possibly send positive voltage on the bias from one shorted tube? If this is the case, a tube tester may not show the defective tube either. A few years back, I had a new tube glow orange and show good on two testers, a Hickok and Eico 667. They exchanged it and put it back on the shelf. I ordered more, and got that same tube back. I returned it, they put it in an amp they had, and it glowed orange. The testers( theirs and mine) would not give enough voltage to show the defect.
have you sent an email to Paul describing this problem? Whenever I corresponded with him I received a reply with "probable cause" explanation from him within hours. I ended up sending min in to him once when it was blowing tubes. He fixed it and added some updates to it for a reasonable charge (less than <$100 as I recall including return shipping)
Sounds like you have a local shop to make it easy. If something is wrong with the bias, it might keep getting worse even with the EL34 tubes in it. You could be in the other room if the tubes started to overheat and cause damage to the transformers, or something else. It could get expensive, plus the parts might not be available anymore. Please let us know what was wrong. Thanks.