Used tube testing is a crap shoot. An old tube can measure perfect and be at the end of it’s life the next day.
Is this true? I always operated under the assumption that tubes rarely die quickly and instead, they just slowly get duller and duller. I realize a tester may not always be definitive on how much life is left on the tubes and that your ears really are the best measure. Although over several years you may become accustomed to the very slowly diminished SQ so the degradation of SQ may not be immediately apparent over time.
My approach has been to replace my tubes after several thousand hours and see how the new tubes compare. If there isn't much if any difference then I put the old ones back in and milk out the rest of the life. But I always thought it might be handy to have a tester just to get an idea whether the tubes may have much life left. Perhaps a tester for that purpose is worthless?