Do tubes have a break in period?

I am not sure about the answer I am giving, but I will take a stab at it. I would say most likely the innards of the amp would need to break in more than the tubes, such as caps, wire, resistors, and so on. Hope I am closer to answering the question as possible.

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.

What I have found with both Valve Art and EH KT88s in my ASL Hurricanes is that they take a couple of days to stabilize bias current in a push pull amp. In that two day period, the bias fluctuated as the tubes burned in. This you need to watch closely for a few days before business as usual. I have noticed zero change in sound if kept biased within tolerance. Those who say they "warm up" are probably experiencing ear burn in rather than tube burn in - a phenomenon that makes us think it is getting better, but we are actually training ourselves or becoming acclimated to the new sound.

I had a V12R, Cary recommends 100 hours.

Yes af they are really NOS 50hrs.

Though the internals of the 300B tube are larger than those of a lot of other tubes; the same components(cathodes/anodes/grids) & dynamics are present in all: (http://www.stereophile.com/content/search-perfect-300b-tube-page-3) (http://www.stereophile.com/content/search-perfect-300b-tube-tale-burn) Read the follow-up of the article as well, for further notes on burn-in(especially paragraph three).

I think there's no definite answer for this question. Time is variable.

My experience coincides with Dan_Ed, in the 20-30 hr range.