I'm sure this topic has been discussed at great length in fragments all over the forums, but I can't seem to find a good thread that centers around it. Though I feel like I've been a gear-addict for ages, until about a year ago, I wasn't gigging regularly. My amp(s) didn't get cranked up ever, let alone for four hours a night two nights a week, so wear and tear wasn't really a concern. It has occurred to me recently that I actually have never experienced bad tubes. I don't know what to look for or what they sound like when they're starting to go. In my searches for information on this topic, I've come across descriptions of microphonic tubes, and I've found a couple preamp tubes that make noise when tapped lightly. But other than that, I'm not really sure what to look for or what to expect from my tubes as they get older. Would diminished headroom be a potential symptom of bad preamp/power tubes?