Vacuum Tube Break-in

Your link has nothing to do with tubes.

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 significant...it 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.

Jerry

In my experience, it can vary greatly from a minimal discernible effect during the first 50 hours to multiple drastic qualitative changes over the course of the first hour or two of usage.

But tubes get hot and metals change when they glow.  I do believe tube burnin is significant...it isn't always positive.  

Agree with @carlsbad and I'll add that the circuit and the entire chain may determine if you hear these changes. I use all NOS or "tests as NOS" and do hear changes in tube sonics, I trust my ears. 

I did try new Linlai 6SN7's which have a 200 hr. burnin (according to manufacturer). I did hear many changes from bad to worse to better. After the 200 hr. mark, I didn't like their sonics even though they had improved.