KT120 longevity

Yea Wolf, just keep an eye on those electrons and you'll know when to take action. Those little buggers will start acting up and start hissing and whining and moaning under their breath. After that they will quickly turn into arsonists. I think those electrons are what the communists sent over here to keep a spying eye on us, so don't let 'em get away with anything evil.

I've read 5000 hours.

I guess you will be the standard case to model future behavior on.

FWIW, I drive my ARC VS-115 with KT-120s. ARC still recommends replacement at 2000 hours. Do they have a longer life? Dunno. Maybe yes if you drive the tubes with quality electrons. I'd call the factory and ask the techies.

Tube amplifier manufacturers typically specify 2000 hours as the useful lifetime of these power tubes. It doesn't mean they suddenly drop dead after that, but performance degrades with more time.