I find that our ears get used to the loss of tone in our amps and you really don't realize it till you throw some fresh ones in there. The guy who has been using the same tubes for 8 years should listen to some fresh tubes and see if he can hear a difference. Mine usually started going away after 2 or 3 years.
12 in 20 years doesn't seem very good to me. The big selling point of NOS, if you listen to the NOS sellers is the longevity. I have to say, I used to run RCAs (preamp tubes) in my Twin Reverb Re-issues and I had more microphonic issues with them than I did with current production. They did sound good, however. I used the 7025 and 12AT7 (reverb) in conjunction with Svetlana (before they were called SED) power tubes. Again, sounded great, but my experience did not support the "better quality" claims.
That's a pair every 3.5 years approximately...I don't think that's too bad for power tubes.