How do you know when to replace tubes

Wolf - Seriously? 8000 hrs on power tubes?? I'm impressed.

Regarding 6H30s...did recently replace some Sovteks in my pre-amp with EH
gold pins. The Sovteks had hours on them but tested strong. Noticeable
improvement in sound quality with the EHs installed, however. I wasn't
expecting much and was pleasantly surprised given these are out of the same
factory, gold is just cosmetic, blah blah blah. Very reasonable prices on the EH
6H30 gold pins too.

Maintaining a listening log is a good way to keep track of hours on power tubes.
When they go, they can do damage.

I didn't count the hours on my 120s but it was about 2 and a half years of
use...even 5 hours a day (I have my rig on more than that usually) is over 5000
hours, and they still held their bias and sounded fine until that last couple of
weeks.

I really AM impressed, WG.

When it comes to power tubes, I'm more concerned with catastrophic tube failure than SQ. Following the recommended tube maintenance schedule is dependent on the amp and manufacturer.

I've had power amps where the tube just gracefully dies but with ARC, I'm definitely following their recommendations.

Heck, almost any triode power amp can log 10,000 hours on the power tubes, the exception being the 6C33. We had one of our amps log 22,000 hours on the power tubes before we finally had to replace one (there were 16 power tubes in that amp, FWIW...).