How do you know when preamp tubes go bad?

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?

you know preamp tubes have gone bad when they call you at 2am asking to be bailed out of driving a solid state amp...

some humor never hurts...
this is not intended to stir tubes/ss shenanigans...to each their own