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.