When it sounds like a tube amp needs rejuvenating,

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!