Intermittent problems are the hardest to troubleshoot. 25 years of trouble-free use is good. Probably retubing should have been done anyway. About time to renew the capacitors too. Either spend for repairs or spend for another. This hobby is expensive. Sell it inexpensively stating the problem/s that you are having. At least get your money back for the retube.
You learned the hard way. Preamps are notorious for unstable startups and power downs. Can take out your power amp and or speakers. Some mute its output on startups, but not all. And their circuitry can fail. Get into the habit of Turning On your preamp Before your power amp. Turn OFF your power amp before turning off your preamp.