@oddiofyl that's right, testers are great for weeding out shorted or bad tubes, not so much for tube life or even for matching. Testers don't test tubes at real world operating voltage and current.
DON:T WAIT! Replace Those Old Power Tubes!
For several years I've cobbled together the required quad of 6L6GC power tubes for my Raven Audio Integrated amp. I say 'cobbled' because out of two sets I culled four which would perform what I perceived as reasonably decent sounding. Being on the last best four I feared one more failing tube would put me out of the music. So, I ordered another quad from Raven Dave (Thomson). At first I thought to just saved them for an emergency but on a whim I decided to just put all the Chinese New Old Stock (NOS) in place. APPARENTLY, and obviously, my smorgasbord set in place previously had various quirks including but not limited to: hum, occasional buzzing, and a bloated`slow tone. My wife immediately stated: I don't hear anything in the background. Indeed there isn't anything but dead silence. NOS tubes are always a "Russian roulette" deal but their unique and amazing "real-ness" is worth the gamble. (IMO) So if you have been waiting until you absolutely have to, DON'T WAIT any longer. The improvement is profound. These tubes are as old as I am! Only difference is I've not been in a box traveling around the world for over 60 years!