I'm sorry, but "burn-in" is placebo nonsense that isn't provable at all.
Why don't you take a listen to the actual R&D Manager at 64 Audio as he recently commented about this topic
in this video (at 7:41). He has every reason to give some crap answer ("oh just listen to our IEMs for 500 hours then you'll see they sound amazing!") but instead he flat out says that the first 5-15 seconds of listening is the best judgement you will have. And this is a guy on the engineering team who actually knows how the drivers work. Listen to the part where he talks about how what you ate can literally affect the sound you hear - now how do you know that your IEM drivers didn't actually "burn in", but instead you are comparing the sound under two periods wherein your literal ear-drum physiology is different?
For those of you who think you can actually hear the drivers "loosening up", "becoming more cohesive", or whatever else: convince me that you're not just priming your brain to EXPECT to hear a difference. It's not complicated: if you convince yourself that the IEM's will "sound better" over time, then you are priming your brain for an expected result. When your brain expects an outcome, it tends to magically "find it". There are so many examples of this very same phenomenon that have been proven in multiple arenas. For example, when MSG became a hot-topic discussion as a flavouring ingredient in food there was an inaccurate initial study which suggested that it would make you feel bad if ingested. Once that "study" caught on among the public, magically everyone and their grandmothers proclaimed that eating food with MSG added was giving them headaches, etc, etc. The guy who initially did the study came back years later and redacted it - his conclusion wasn't even accurate. But yet, because people so thoroughly believed that MSG would make them feel bad in some way, they convinced themselves that eating Chinese takeout was giving them headaches. This myth is actually still active - there are people still convinced that they feel some unexplained ill effects after eating even mild amounts of MSG.
These phenomenons are well studied. Just like when they give someone who has some unexplained ailment a sugar pill but tell them its some magic new medication, and the subject all of a sudden doesn't feel whatever ailment they were complaining about.
The reality is that everyone wants to justify the IEM that they potentially spent hundreds or even thousands of dollars on. "Oh when I first put them in I felt they were pretty good, but then after 100 hours of burn in they became amazing!". "Oh they're pretty good with the stock cable, but this $400 cable (which is just transmitting data lmao) made everything about them better!". Nah dawg, you're just coping. It's pure cope to believe that the tiny drivers in IEM's need hundreds to hours of operation to start working as they should.
After-market cables are the equivalent of thinking that slapping on a $20,000 body-kit on a Honda Civic is going to all of a sudden improve its acceleration.