After I read that I was shocked that standby switches are still commonly used.
If I was still gigging, I'd still use the stand by switch.
ALL the blues guys that I hung around with/saw play in Chciago, that played Fenders (most did) or another amp if it had a stand by they used it. And they had always used it since the 50's. Many of them were big names on the blues scene in the '50/'60's. Guys that Clapton, Page, Jimmi H., J. Beck, etc..... loved.
I
NEVER saw a Fender amp go down on stage (although I'm sure it happened) and their amps/tone was fine. So was my BF SR, and I played a lot with that amp.

I'm not trying to say Merlins wrong but if the tubes were being affected to the point of the sound degrading and/or dying they would have heard it/known it. They made their living off of playing and they were too many of them for them all not to hear it and/or figure it out.
I did have a 6L6GC's heater burn out on my BF Dual Showman (1st amp) but it still played. I could hear that something was wrong right away but didn't know what it was. A guitar player came by our little gig who played for Buddy Guy and Jr. Wells for years and he sat in and played through that amp with the bad 6L6GC. When he came down I asked him if he thought my amp sounded funny, he said it sounded fine.
My BF SR lost a faux heater CT 100R, at home, changed it and the bad 6L6GC, worked fine again for years.