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.