BHK250 Tubes

I’ve had my new 250 BHK since around May 2024. At an estimate of 15 hours/week, that’s probably approaching 1500 hours; possibly much more. I few days ago I noticed a loss of gain in my left speaker. I suspected my pre (ARC LS-3) and pulled it out; it tested fine. Today, I reconnected the system and there is no issue.

Clearly, it’s about (or past) time to replace the tubes; however, I’m wondering if there is any reason that the temporary loss of gain could be related to the age of these tubes?

I’ve never experienced a loss in gain when the tubes in the BHK 250 age, just a loss in vitality.