How do you know when preamp tubes go bad?


Over the last few days the sound quality of my system has diminished considerably. I'm using a tube preamp driving a SS amp.I've seemed to lost a lot of my my soundstaging.Any input would be greatly appreciated.
mfb33
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