The day after I started running in at 15%, electronic music sounded worse on any volume level I heard it at. It had less imaging and definition (if I'm using the right word) and wasn't enjoyable. Maybe it was also missing some detail and soundstage. And when I started running in at 16% volume 2 days ago, it sounded even worse
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Is this kind of change in sound normal when running in a headphone at loud volume for a long time?
To add to megabigeye's post:
1. Is there even obviously bad sound output like crackling etc?
2. By "running" do you mean you just left them running with some listening? How much of that was actual listening?
Because if the drivers are damaged either you get zero sound or you get undebatably bad sound, not something that can be perceived as minor changes. For example if you modify a car's engine and the exhaust note sounds different, does it actually sound like it's backfiring or the RPM drops too far down when off-throttle, etc, because otherwise regardless of your tone preferences forced induction on an engine can make the resonant frequency on-throttle higher or lower depending on the engine and forced induction type used. Your screaming banshee of a Honda or Ferrari having a kompressor whine in the background while getting throatier doesn't mean it's malfunctioning, nor is a deep gurgling BMW that moves a bit from a growl to a scream even more once you're past 5000rpm.
Without "obviously something's wrong" kind of sounds what you described there so far sounds more like break-in, and on headphones, two things break in: the transducer mechanical parts and your earpads, as there are several ways to interpret those descriptions. "Less definition" and "missing detail" in what, exactly? If you're referring to higher frequencies then a broken-in driver and maybe compressed earpads can have more bass performance. The same sounds are still there, you're just getting more of other frequencies, either right out of the driver or how the sound interacts with the environment past the diaphragm.
Similarly, "imaging" and "soundstage" are already understood erroneously by many from the start. For example many just notice the HD600 has a small soundstage, but then some of such people exalt Grados having a "wide soundstage" with "3D sound" when these are actually "why are the cymbals set forward where the vocals are and to the flanks past where the guitars are? is Mr Fantastic a drummer now?" and "why does the bass drum sound like it's coming from the vocalist? does Iron Man have a robo drum in his armor when he sings?" respectively (and I'm saying that as somebody that enjoys both an HD600 and an SR80e, and used to have an SR225). So we have to flesh out more details about what you're actually hearing as well. Do the vocals sound deeply buried in the rest of the mix? That could just be the low end going; also this gets worse if you're running bass restoration software on compressed files (like Alpine's Bass X-Pander) where they just blanket boost everything based on how compression can trim bass regardless of how much should be there in the original mix. Similarly "lower detail" can just be auditory masking or even just "more balanced, at least as intended by and achievable to the engineers" as the low end catches up, kind of like (but not the exact same dynamic as) a cold tube amp that sounds like a cheap radio (very generally though if you need an hour to warm it up it's either your room is cold AF or the circuit sucks in getting the tubes ready).
If you use it a lot in that period, it will accelerate the latter, and you have both variables kicking in together;
however it shouldn't seem this drastic a change if you've been using it enough as the changes due to the earpads getting more compressed will build up gradually, so you'd be less likely to notice the change as drastic even if driver break in tends to have a more drastic kick-in. If you haven't been using it much, you might not have earpad wear as enough of a factor, but then any changes can be more sudden to you when they kick in and you think your damaged something, unlike one speaker and amp (new and NOS) I was breaking in and mid-song sounded different (went back to the start, sound really was different) as it went from "is this it?" tin can to "oh finally it sounds like a concert," but cleaner ie low noise and not standing in spot with bloated bass or chest beating bass drums.