I am pretty sure it was mentioned already and if not, everyone was thinking about it.
Sounds like right now it boils down to:
- Anedio D2 (no DSD, specific treble hot signature, not sure about USB input quality)
- M51 (too big, no DSD, USB is not the best)
- Linx Hilo (only DSD64, not sure about USB quality, useless for most ADC but you pay for it)
- Yulong DA8 (full DSD/DXD support, stellar USB, preamp function, not too shabby integrated amp. Comparison review with Hilo are mixed, some state that SQ difference virtually non existent, some greatly favor Hilo. Even if HIlo sounds like 5% better, DA8 is almost half of the Hilo price)
For now I ended up with Yulong but keep eying Hilo as a possible step up.
The thing I love about the Hilo is NEUTRALITY. It is not cold, clinical, agressive, ruthlessly revealing, unforgiving, nor is it warm, forgiving, tubey, golden, or any of that. The sound lacks grain, and instrumental timbres are right. I am not comparing it to the others mentioned -- I've not heard them -- but it's the best DAC I've owned (though not the most expensive) in 25 years of listening to digital. If you are interested in classical -- or other unamplified -- music and a frequent concertgoer, as I am, and your ears are anything like mine, its neutrality will be apparent. I find it like drinking from a clear mountain lake.
Prices: Yulong, $1300 on Amazon; Hilo, $1850 Head-Fi price.
The Hilo's headphone amp has, I believe, a 0.08-ohm output impedence, so it should not be affected by impedence of the phones you choose.
But, in the end, DACs are really a matter of taste, and features do vary between the DA8 and the Hilo. So maybe you can try some before you decide?