> I noticed a difference in clarity using Audio-Technica ATH-M50s and Beyerdynamic DT 990 250Ω. Specifically, the Beyerdynamics could be driven "good enough" for me with the iPhone SE jack, but I wasn't happy with how they sounded on the Lightning dongle.
Scientifically speaking, it should be noted that the adapter performs exceedingly well to the point of audible transparency[1]. You are not going to hear the distortion components as they lie below 90 dB.
The fact of the matter is that your ears simply cannot be trusted, not when it comes to discerning differences in DACs and amps. There is a beautiful read[2] that illustrates this by NwAvGuy, it's worth checking out even for non-audiophile.
And Xiph's Digital Show and Tell[3] (again, a must-read) tells us that the 44kHz of the dongle is more than enough for audible transparency to humans.
The differences you hear are likely due to incorrectly matched volumes. If you volume-match to a fraction of a decibel, you probably would not be able to tell the difference in a blind test. There are other potential differences like a different impedance etc but these should only effect a very small subset of IEMs, let alone your headphones.
[1]: https://www.kenrockwell.com/apple/lightning-adapter-audio-qu...
[2]: http://nwavguy.blogspot.com/2012/04/what-we-hear.html
[3]: https://wiki.xiph.org/Videos/Digital_Show_and_Tell