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The effect has been measured: https://www.head-fi.org/threads/shure-se846-impressions-thread.675219/page-1090#post_12909677
The difference is a couple of decibels, unless impedance gets really high. Some people notice a 1 db change at one point in the FR, some people don't. And some people may prefer the resulting sound anyway.
I mentioned that an output impedance of 1.3 ohm would be a deal-breaker for me, because I can notice a 1db change in a fairly narrow range as I A/B in EQ and I think the 846 needs some help in that range anyway.
I don't think those are measurements. It looks like Thomas has just calculated the gain ratio and converted to dB offset in Excel or something. Still, this proves the point and saves me a ton of work at the weekend, so thanks for the link Thomas knows what he's doing and his numbers look right. With a 1.3 Ohm output impedance into a frequency where the SE846 was ~5 Ohm, you'd see ~2 dB dip, and at 8 Ohm, ~1.3 dB dip.
I completely agree that these deltas could be audible and, of course, a smaller output impedance would be preferable, but "deal-breaker" still sounds a bit harsh. There are very good dedicated DAPs out there that have similar specs (FiiO's excellent X7 Mark ii has a 1.2 Ohm output impedance). Perhaps it's happened, but I've never heard of a deal being broken from an iPhone showing a 6 dB dip in the SE846's mid-range. Perhaps, as you say, this is because many actually like the change(?) The impedance dip occurs before the ~9 kHz resonance peak, so it would actually help those that like V-shaped signatures. The range where I feel the SE846 needs some help is 9-10 kHz and up, and its impedance curve does (relatively) lift its upper treble with a higher O.I. source. Of course, it also adds some low-end, but plenty of people like that too. The effect of a 1.3 Ohm output impedance is still way less than you'd experience from changing filters:
https://www.head-fi.org/threads/se846-filter-mod.802350/page-5#post-13200821
or even changing eartips:
https://www.head-fi.org/threads/shure-se846-impressions-thread.675219/page-1205#post-13617865