I will just throw this out there that the notion of impedance mismatching for lower or higher impedance headphones might be an overblown/over-emphasized consideration. I have now been in two prolonged situations where I had mismatching that if you listen to the threads around here on the subject would lead you to believe that if your equipment is mismatched your sound will be significantly impacted for the worse. First time round I had a Valhalla and HF2 combo for about a year before I sold them. Loved the sound quality and I could not at all detect the horrible effects of the impedance mismatching (except perhaps in one very specific case, but I couldn't confirm the link). Currently I am using my Auditor with almost a 10 ohm output impedance with my Denon D7000s which of course are low-impedance cans. The sound is stellar across the board, really poised, detailed and controlled up and down the frequencies. As far as I can tell these differences in performance due to impedance mismatching are measureable, but that doesn't mean they are audible. When I was first considering the Schiit Valhalla Jason and I exchanged several e-mails on the subject and he told me that he had several clients that were music producers and sound engineers that actually really liked the Valhalla with low-impedance cans.
There is also likely a difference depending on the specifics of the gear. For instance, as the frequency response of the D7000 is fairly tight and not all over the place it would likely not be as impacted by the impedance mismatching anyway, plus the characteristics of the amplification matter as well. All of that is to say that simply saying that a low-impedance headphone will sound worse on an amp that is higher in output impedance is not always going to be true, and even when it is true, just how much impact on sound quality are we really talking about here? I know this is only quasi-related to the original thread, but I felt it was important to mention as the thread adds more weight to the notion of impedance mismatching which I feel is more nuanced and less black and white then it is represented as being. Sorry for thread hijacking, my bad. I am actually considering getting a Ray Samuels design someday so that is why I read threads on his work such as this one.
I have to disagree.
you take 2 examples that are monodrivers with pretty stable impedance across frequencies. so the bigger change will be damping that you might or might not notice (or you might even prefer a little less damping, like some do for loudspeakers). on the denon you will get a little sub-bass boost with higher impedance source. as for the the hf2 you will mostly get a boost in high freqs (and that should be clearly audible compared to <1 ohm outputs).
but I agree this won't be a life changer on both phones.
on multidrivers you would get some nasty frequency changes due mostly to crossovers.
it is good to trust experience more than forum chit chat, but your case is far from covering all possibilities mate.
people should be wary of impedance output anyway, there is no bad side effect to a low impedance output (at least sound wise) . when the opposite if not well informed can be a gamble and turn a nice phone into beats.
to get back on topic, here the question is more about knowing if AmberOzl's IEM will mostly need voltage or current and has nothing to do with the amp's output impedance.
also I'm confident the lightning's output will be quite low(at least on low gain) and won't be a concern for any phone.