How do you know when preamp tubes go bad?

I pretty much agree with Mezmo's observations, and would just add that when the 6 tubes in my pre-amp (SF Line 2SE) get to be 12-14 months old, music quality starts getting a bit hard, bright, strident, and progressively more fatiguing. I keep the pre-amp in standby when not in use, and this uses up some tube life, and then I listen to music 3-5 hours per day.

With new tubes, the hardness and slightly fatiguing music character goes away. I've used this pre-amp over four years now and have changed tubes several times (not counting experimanting). Perhaps this (short)1 year tube life has to do with the stock Sovtek 6922 tubes I had been using. BTW, I recently switched to a set of NOS Sovteks and realized a significant improvement in music quality-- hope they last longer than the stock tubes. Cheers. Craig

Usually the sound gets thin, dull, and lifeless, when tubes are nearing the end of their useful life. You lose dynamics, the bass get soft, and loose, and the highs seem rolled off. Most times, it is so gradual, that you don't notice, till you install, new tubes.
Sovteks usually last for 1-3 years, depending on how much you play them. NOS types, tend to last 5-10 years, under the same circumstances. I think most NOS tubes, are sonically superior, by a long shot, to Sovtek tubes.

I'm no expert, but I thought they started to "ring" at the start of their demise. The ringing would go away by tapping the chassis top but randomly return weeks later. I'm currently running an Audible Illusions 3A with white Amperex 6922 (NOS).

Watch for the upcoming FOX TV special,
"When Good Tubes Go Bad!"

I read somewhere that Victor at BAT said an old tube can be reactivated by feeding it a certain current at certain voltage. Does anyone know how to do that?