Vacuum Tube Break-in

Is there such a thing as tube break-in? I been laying with many tubes in about 20 tube amps for 40 years and never heard a difference in a new tube vs same tube with 50 hours use. I always bought and used NOS tubes, not used. I believe if a difference is heard with break-in, it is the placebo effect.

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Yes, but how much sound difference there is varies with each brand and type, and is largely effected by the rest of the system, room, music source, and listening habits.

I think burn-in is greatly exaggerated, especially in passive, not electronic equipment.  But tubes get hot and metals change when they glow.  I do believe tube burnin is isn't always positive.  

I put in a set of Russian tubes last week and they sounded best 10 minutes after I installed them.  Still good but I liked them best then.