I once owned an all-tube phono preamp (Icon) and only gradually noticed its sound no longer seemed up to snuff. New tubes changed that dramatically. BTW, the mfg. admitted the tubes were driven hard in that application.
As far as tube changes making little difference, I have to emphatically disagree. Changing out KT-88s for KT-150s in my Primaluna Integrated some years ago gave terrific power but dramatically sacrificed timbre. New preamp tubes in that same amp turned the sound so crisp I sold them nearly immediately (failing to let them break in, alas). More recently, in a Vincent tube integrated amp, I could not get the highs in my rather live room to settle down. New tubes not only retained the high-end and made it more natural, they also dramatically filled out the bass. None of these sonic changes were even subtle!
As far as tube changes making little difference, I have to emphatically disagree. Changing out KT-88s for KT-150s in my Primaluna Integrated some years ago gave terrific power but dramatically sacrificed timbre. New preamp tubes in that same amp turned the sound so crisp I sold them nearly immediately (failing to let them break in, alas). More recently, in a Vincent tube integrated amp, I could not get the highs in my rather live room to settle down. New tubes not only retained the high-end and made it more natural, they also dramatically filled out the bass. None of these sonic changes were even subtle!