Yeah, that's what I was hoping. And I just looked, I don't have an old box of 6L6's with a date on them. So I think these are the tubes Alan put in it in 2016.Probably power tubes going south. Even when you are playing softly, all the tubes are getting the full plate voltage. It still causes wear. Also make sure if you are using the loop that you have good cables and connections.
BTW, yup my Tucana 3 runs 6L6's for anyone interested(6V6's and EL34's too).