Sounds like you already know what you need to do; replace those tubes - cheaper than a woofer!
Help! My tube preamp shocked my speaker!
hehe. Actually, what happened was one afternoon (the stereo had been playing for several hours) I heard a wierd buzzing/flickering/crackling sound, very briefly, but quite loudly, eminating from 1 speaker. I immediately walked into the room, just in time to see the woofer of my left speaker moving in a very srtange manner, as the noise ocurred again. I immediately shut the amp off. When I turned it back on, it happened immediately. So, I turned the amp off again, followed by the pre. When I turned it back on this time, everything was OK. Hasn't happened since. One of my 12au7 tubes (there are 2) is glowing brighter than the other, and actually started to soften and melt, however slightly, a silicon tube damping device that I purchased from Audio Research. This noise happened to me once before, but it was another tube preamp, which I had just bought, and was immediately replaced by the manufacturer. What's the deal??? I need to burn in some cables, but I'm not leaving my preamp alone with my speakers, I just dont know what she's capable of doing...