Seems like the best "upgrade" for tube amps....

'Is there a technical reason for this?'
Technically, I call them "operating temperature". as all products (human & car included) were engineering, designed and tuned at such degree to its best.

If I let tube guitar amps warm up (for 30 minutes?) before using them I would make a lot of people stand around. My experience recently with a quad of kt120s was that running music through them as soon they were ready (30 seconds or so) caused the tubes to last about 7,000 hours, which was more than I expected.

Hi Wolf-Garcia, WOW, 7,000 hours !! What kind of amp are you using them in ?

That's good to hear about KT-120s. I have been using my pair for over a year so maybe I have a few more years to go. Mine are run pretty conservatively----500v and 65ma---so that should help.

By the way, I don't see any point in waiting very long before playing music through the amps. I usually let mine warm up a couple minutes (while I am sorting through LPs) but that's it.

Interestingly, a few hours of warm up applies to solid state gear as well. In fact the manufacturer of my solid state preamp recommends at least two days of warm up before listening, then leave it on continuously, turn it off and you will start over! I believe that these warm up periods involve the electrons finding their shortest signal path as well as transistors or tubes reaching their optimum temperature.