DAC's, Audio AMP's, RF AMP's, all manor of digital and analogue devices change characteristics as they "age" - or burn-in.
I learned early on as an audiophile to not "toss out" a new piece of gear just because it's not sounding good when I first power it on. I give it as long as the return period allows, running it day and night - enjoying it by listening to it, and most of the time it recovers it's expected graces and I keep it long term.
As a consumer I've experienced it every time I buy a new device and live with it day after day. I've noticed this burn-in effect since the 1970's, and I'm still hearing it today with my FiiO M15 - it's sound stage has improved and "mellowed out" - it wasn't harsh, but now it's even easier on the ears 2+ weeks later.
Nothing's changed over the years, except now more people are experiencing and recognizing it themselves.
Including Hi-fi Audio reviewers:
HI-FI AUDIO COMPONENT BREAK-IN (BURN-IN): FACT OR FICTION
John E. Johnson, JULY 29, 2019 EDITOR'S DESK EDITORIAL
"...I have increasingly become aware of an actual difference in the sound quality during my product testing. And, I mean, I could really perceive the difference. It is dramatic.
One DAC, for example, came with a recommendation of leaving the unit powered on for at least one day before making any critical judgments. Initially, I did not think about it. But,
when I first turned it on, I noticed that the sound had an unpleasant harshness. After 8 hours, the harshness was reduced, and after 24 hours, it was completely gone. The DAC had a much smoother overall sound. It was dramatic enough that I decided to write this editorial.
What is the nature of audio component break-in (burn-in)?
As a scientist, I want to know how this process occurs..."
https://hometheaterhifi.com/editorial/hi-fi-audio-component-break-in-burn-in-fact-or-fiction/
As an engineer I've had lots of experience with age related differences in electronic components - with ECO's and debugging / troubleshooting notes for technical staff when repairing equipment that is returned over decades of use. There are definite cycles of aging - and different reactions in different climates to component aging. But you say that's on the scale of months, years, and decades, what about initial burn-in - say in the 100 hour range?
That's what "burn-in" in manufacturing is used for in more critical applications of electronics. Those devices aren't simply assembled, tested, and packed up for shipping like consumer equipment. Nope. There are cycles of heating and cooling along with ambient temperature running of the components in board level and system level testing.
You don't hear about that because it's something not usually done for consumer equipment - the process is too expensive.
So for consumer gear it's your responsibility to do the "burn-in" testing - and as part of that you will hear audible differences in the sound produced in many gear. It's variable, but always there to some extent. A lot people don't notice it because of their own physiological variations from day to day will confuse or mask the aging sound of the gear through their ears.
Anyway, if you hear it great, if you don't hear it that's also great - whether you can perceive it or not, it's there either way so give your new gear as long as possible to "burn-in" before giving up on it.
As an aside, before I could "hear it" I could "perceive it" as "easier to listen to", not "getting on my nerves", rather than wanting to turn it off I want to keep listening to it. That's when I know it's turned the corner and burn-in is rounding the corner.
That happened with my M15 last week, and I turned around and decided to keep it. It now sounds too good to give it up, and I don't want to stop listening to it.
I fully expect the same burn-in audible improvements to happen with my new A90 and D90 MQA...
