Do tubes have a break in period?

From my limited experience, I've found that they have a relatively short break-in period -- a day or two max. I did not, however, conduct listening tests for high output tubes but don't suspect it's too much different.

I tend to agree. Most I've tried have sounded their best after about 20-30 hours. If they don't by then, they probably won't ever.

Yes,tubes need a burn-in period of at least 48-hours. There is no need for a signal input. At the end of the burn-in period, the tube will have a better signal to tube noise ratio.

Yes they do.

They do, and I burn in new power tubes for 48 hours and then check the bias at 48 hours one more time because they seem to settle in at this point and the drift after that is minimal, at least for the KT77s/El34s I am using.