When it sounds like a tube amp needs rejuvenating,

Power & output tubes going bad tend to snap crackle & pop,literally...
Small signal tubes going bad are sneaky,usually only displaying lack of volume until going dark...JME,YMMV...
PS:Figure around 5000 hours for small signal & between 2000 & 3500 for power/output depending on bias...

thank you - to the point - I appreciate your experience. 

- john

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!

Just change the output tubes first. Listen for SQ. Don't forget to adjust the bias voltage (if your amp isn't cathode biased). The small signal tubes may still be fine!