While doing research for my recent rig upgrade, I read somewhere that tubes are expected to last only one to three years (SWR website, IIRC). Mine is at least seven or eight years old, but I don't play terribly often (I'm not a working musician, just a weekend warrior). Is it a good idea to change a preamp tube periodically even if it's still working? Does the tone change or degrade with time? How can you tell if your tube is going bad? I was under the impression (for no particular reason) that you use a tube until it dies, then replace it... that's what we used to do with our TV (yes, kids, I'm that old). Related questions: What does "microphonic" mean? What's the consensus on the best tube out there? Thanks, and apologies if this is all covered in a FAQ someplace. I haven't found one.