First, I did a search and looked through all ten pages and didn't see anything which answered this question. I recently received a Musicman Audiophile HD-500 amp from our very own Planet Boulder, thanks John, and while the amp is the SHIZNIT it has an interesting property or two to it. First, if I tap the outside of the amp, the casing, my cabinet makes a tapping sound. If I hit it harder it makes a louder tapping sound. Sometimes it makes a rattling type sound even though I have the amp sitting on top of my BAM as opposed to my Avatar so it's isolated pretty well from the cab's vibrations... is my poor 12AU7 microphonic? I have an Ei I can replace it with and check it out but I'm just curious what you guys have to say. Also, figure I'll just replace the tube with a few options and see which I like best since they're so cheap... anyone got a favorite 12AU7 for a bottom and midrange heavy tone? I figured I'd try a JJ and an EH, maybe an old RCA... the one in it now is a JAN and I really like its tone but if its microphonic it gots to go. Maybe another JAN then... so many decisions so little money.