When it sounds like a tube amp needs rejuvenating,

oh also - when a tube can no longer hold bias, it's time to replace it., right ?  

-john 

Short of catastrophic failures, tubes that are past their prime can sound “tired” or lackluster; you start noticing treble doesn’t have the life and sparkle you were used to. The best approach is to buy a new set of tubes before they are needed, then swap the new ones in periodically to see how they compare to the old set.  If you don’t notice an improvement, put the old set back in.

thanks - yeah, I've noticed a lack of air  and space around individual instruments - in other words, the 'magic' I love about my system. 

Part of the tube amp "thing" is the fun of collecting tubes and trying various ones in different combinations...many tube heads do this and wind up with a lifetime supply to mess around with...I agree utterly with swapping in fresh tubes being the way to absolutely tell that tube retirement time is here. I usually have a small ceremony when retiring a tube...cake...small certificate...

When the noise level behind the music becomes noticeable! Power tubes usually go first. Small signal tubes can last way longer - 10X that of output tubes!