[TrantaLocked]

Preferably something like the Etymotic 75ohm adapter, but one in 2ohm and one in 10ohm.

Edit: I actually already own a volume adapter that works: https://www.ebay.com/sch/i.html?_fr...male+to+f+3.5mm+stereo+audio+adaptor&_sacat=0

BIG edit: Apparently I was wrong; it seems you need a LOT MORE than 10 ohms to get close to what I'm hearing from the D3. The increased impedance will increasingly up the 2-3KHz range even compared to 10+KHz. This makes me believe that wear of the D3's headphone jack has caused its output impedance to increase far above 10 ohm over the years.
So then, I tried my ER4P -> S 75ohm adapter and that immediately sounded a lot more like the D3. However, it seems you can still go even higher than that as even at 300 ohms on the volume adapter, it still sounded good. So the RE400 just seems to really like impedance. But I think after hearing with the Etymotic 75ohm adapter, which actually reads at 80 ohm on my multimeter, I'm still not 100% if that's an overshoot or not far enough, but right now I suspect it's a slight overshoot and the correct impedance to match my D3's sound is maybe 50-60 ohms from the EarMen Eagle.

If the hypothesis of wear increasing impedance of the headphone jack is true, then it could be right to assume that the impedance of any headphone jack including those on the EarMen Eagle and HiBy FC3 will also increase.

I measured with a multimeter. It starts at ~10 ohm at highest volume, then rapidly increases in impedance as you turn the volume down. It does indeed make my near-zero output impedance DACs sound more like the D3, however it overshoots on the treble a bit much, and it's clear that my EarMen Eagle still has its own sound that makes it still a bit bassier than the D3 even with the volume adapter. So maybe a normal 3.5mm cable or a couple daisy chained to add ~5 ohm or something, or a volume adapter that starts at 0 ohm so you can choose values under 10 ohm.