Sounds like a good question to me. And while I've used tube equipment for over 35 years, I admit to not knowing either. What I am reasonably sure of is that for a typical Class A/B circuit, how loud you play will not have any practical impact on the life of the tubes. The impacts of proper biasing and the designer's decision on how hard to drive the tube within the circuit will overwhelm any impact we can have as users in just playing music through them.
.
.