As far as I can remember burn in has been very obvious on all new stuff I have purchased, since I got high end gear… 40 years. Over the last few years I have (for various reasons) broke in about five new pieces of Audio Research Reference electronics and the pattern has been identical. Hugh difference (starting from thin, closed in, limited bass) for about 100 hours… then remarkable improvement, then switching back and forth from good to bad and visa versa around 120 hours… then slow improvement through 600 hours (fastest at first). This time, I did notice improvement over 600 - 1,000 hours… small but noticable… I had read about this before but never experienced it (usually because it was covered up by me switching power cords or interconnects or something.
This has always been a real pain because I usually go through an upgrade cycle… and want to get through it as fast as possible…. But I never buy everything at once so getting hundreds of hours in is hard. I have an extra Schiit system just to break in stuff.