Wellll I only ever really saw proof with the Sabaj D5 with its 9038Pro chip, which could sound better once warmed up. Otherwise, I would find it more plausible the burn in is in your head
I should clarify: I've seen it twice, both with brand new equipment. A period of operation seemed to be required in order to bring them up to speed. Once 'broken in', I haven't heard any on-going warmup issues.
I would point out that I primarily listen to physical, acoustic orchestra instruments, and having played such an instrument myself, I know what to expect in terms of sound. String quartets, particularly the cellos therein are great points of reference: there is the gritty rosin on the bow, and a smooth sleek sound which has to be rendered without descending into a metallic clatter in the lower register.
Some of these YouTube reviewers prognosticate, and then you catch them evaluating with electronic music. Nobody has any idea what these electronic instruments were supposed to sound like because they never physically existed. If the reviewer likes the particular 'sprong' sound of a particular electronic instrument played through a particular DAC, then fine. But don't say the DAC sounds better or worse or there is no difference in the rendition of competing DACS. Garbage In = Garbage Out, particularly in the case of a good DAC.
All, of course IMHO