I tried the EarMen Eagle with my HiFiMan RE400s and compared it with my Audioengine D3. Both sound great but I'm quite sure now that the D3 sounds more forward in the treble than most DACs because of the 10ohm output impedance. Every time I try a new DAC, it's always the D3 that is brighter and at this point it can't just be coincidence that I haven't found another DAC as bright as the D3. It's just the output impedance, as most modern dongle DACs are near zero output impedance with a flat output.
But the RE400 + Audioengine D3 is an absolutely perfect combo. The EarMen Eagle does sound as good if not better in most other aspects, but the D3's output impedance happens to add detail and spacing in just the right amount without sounding too bass light. However, the RE400 still sounds amazing from any DAC, but the RE400 + D3 combo is the best headphone experience I've ever had. This combo actually sounds like an ER4S without the sibilance and with a tad more warmth,
and without sounding dark like the ER4P. I suppose all you need to do to achieve a similar thing on another DAC is to buy a 10ohm adapter. Audioengine says the output impedance on the D3 is 2ohm but online measurements usually show 10ohm, so if I were to try this out I'd buy one adapter of each seeing I don't know which version I have.
Edit: I have the impedance adapter!
https://www.ebay.com/sch/i.html?_fr...male+to+f+3.5mm+stereo+audio+adaptor&_sacat=0
It is exactly this model that I tried that starts at 10 ohm at the highest volume then increases impedance very quickly as you raise volume. I measured with my multimeter and found this result! I tested it and I do indeed hear the difference in treble response,
it makes the EarMen Eagle sound a lot more like the D3 but maybe overshoots on the treble a bit, and still a bit more sub bass than the D3. I think you would maybe want a little less than 10 ohm, something that starts at 0 ohm instead or maybe a normal adapter cable. I have a couple splitter cables that could work daisy chained.
More info here:
https://www.head-fi.org/threads/where-can-i-buy-custom-impedance-adapters-for-2ohm-and-10ohm.964061/
I do think the EarMen Eagle is very slightly more natural than the D3 so in general I would recommend it over the D3, but honestly it's so close already. So yeah, the Eagle does really impress me but the D3 is still one of the best dongle DACs I have heard. If the D3 is a 98/100 the Eagle is a 99/100, besides the fact the D3 is a better fit for the RE400.
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More on the impedance situation:
http://nwavguy.blogspot.com/2011/02/headphone-amp-impedance.html
UE 700 graph:
https://www.stereophile.com/images/ifmeasure/UltimateEarUE700.pdf
RE400 graph:
https://www.stereophile.com/images/ifmeasure/HiFiMANIE400.pdf
Notice how the RE400 has a pretty flat impedance curve but with a hump at 2khz and a slight increase above 10khz? That leads to those regions being less affected by source/output impedance as the other regions get reduced. But for a set with a crazy roller coaster impedance graph like the UE 700, you will get a more extreme change in treble response with the D3. But the result for the RE400 and the D3 with 10ohm output impedance is a very slightly brighter response that to my ears is almost perfect and is the most transparent sound I have ever heard from headphones without using an EQ.
My review comparing the D3 vs HiBy FC3 and EarMen Eagle:
https://www.head-fi.org/threads/hib...p-mqa-compatible.951633/page-12#post-17038274
TL;DR is the D3 is brightest, FC3 is almost as bright as D3 but has punchier sub bass, and Eagle is warmest with more forward upper bass and mid range. Just be aware that the D3 will still be punchier than DACs with weak amps like the Fiio K1, so it isn't like the D3 is a lightweight. It has plenty of power to fully drive most sets' bass. I like the D3 the most for the RE400 but the other two still sound great because the RE400 is just a god tier headphone.
