I've been pretty lucky with microphonic tubes ... not having problems. One of the 6V6 power tubes I have has always had this very slight oscillation noise. Like popcorn popping way off in the distance. It's consistent (always there) and you notice it when the quitar is off or you are about to turn the amp off. If you touch the outside of the tube it stops. It's not really microphonic in that it does not seem to pick up a signal from amp vibration. It's just creating this tiny little rattle underneath the rest of the sound. The tubes seem sound fine ... those little high temp rubber bands wrapped around it would probably stop the noise ... since touching it stops the rattle. Other than replacing perfectly good tubes, any other ideas?