Burn in seems to normally be non-linear and varies between components.
A while back I had the experience of breaking in three identical Audio Research Reference 160s amps, without changing anything else in my system. I was very shocked how each went through exactly the same sequence… approximately but not exactly the same as other ARC components.
In general the sequence was that out of the box they sounded a bit trebly and flat. They would gradually improve for about the first 100 hours. Sounding much better. Then wildly fluctuate between bad and good for about twenty or thirty hours. I mean, when I would turn it on, i never know of it was to be Jekyll or Hyde. Then it would settle down and improve slowly… not linearly but at a diminishing rate through 600 hours. What I was surprised is, while really small… there was a small amount of improvement through 1,000 hours and then rock steady.
I have lots of experience with other components as well. But this tends to be how ARC has been in the last twenty years.