my experience has been as tubes begin to "poop out" you lose a little something sonically. I have noticed this after tubes become and noisy and before as well. I have noticed on both my line preamp and phone preamp.
Tube Life
I’m using a phono pre-amp with a pair of telefunken 12AX7 and one 12AT7. I purchased these tubes several years ago and they were rated at about 50% life. Are tube noise and tube rush the only reasons to change out tubes? Everything is silent and yet I can’t help but think that several years of use should indicate some sort of wear. Any experience or suggestions will be appreciated.