Noises that tubes make as they age

I have had an AI3 pre-amp for a few years and I do like what it does to my 5 channel Carver power amp. What I have not seen posted are some desciptions of tube noises for us novice "tubies". Obviously no sound is a pretty simple. But what about hiss? Personally I have experienced louder hiss and some crackling noises in my right channel with the power amp gain sensitivity turned up full and my AI3 turned up 1/3 to 1/2 volume. Should we periodically use a radio shack meter to measure hiss loudness differences in different channels just with the amp and preamp on with no source playing? How much hiss or noise difference is acceptable? Say if one channel has 60 DB and the other has 68 db? Any other noises besides hiss or crackling that we could learn about?
Other annoying tube noises that I've experienced are line hum, microphonics, parasitic oscillations, plus the hiss that you've already mentioned. And this was with Cary & VTL equipment, even the 5.5 I couldn't stand it. Good reason to go with solid state; I did & have never looked back. I don't *even* miss the wasted power consumption & overheated room in warmer months either.
I had a bad tube once. Being from NH, it sounded like a blizzard, with full wind effects, but not as loud. This was with a brand new tube preamp from Sonic Frontiers. A new tube cured the problem and now there is silence. I have a few tube items and have not had any major problems. I do feel guilty about leaving them powered up though....