Having spent seven months with the HA-1 (as a beta tester), here's my list of OPPO HA-1 chains that I most enjoy, tailored for each of the headphones used (all of them with balanced cables), ranked from most favorite to least favorite:
1) Win7 Laptop > Foobar2000 > HA-1 (USB receiver + DAC + amp) > LCD-2 rev. 1 (for all recordings)
2) FiiO X5 Coaxial Out > HA-1 (DAC + amp) > LCD-2 rev. 1 (for all recordings)
3) FiiO X5 Line Out > HA-1 (amp) > HD800 (excluding poor recordings)
4) Win7 Laptop > Foobar2000 > Teradak Teralink X2 USB-to-S/PDIF converter Coaxial Out (powered by 5V battery or Teralink PSU) > Beresford Bushmaster MkII DAC Line Out > HA-1 (amp) > HD800 (excluding poor recordings, limited to 96/24)
5) Win7 Laptop > Foobar2000 > HA-1 (USB receiver + DAC + amp) > PM-1 (for all recordings)
6) FiiO X5 Coaxial Out > HA-1 (DAC + amp) > PM-1 (for all recordings)
7) Win7 Laptop > Foobar2000 > HA-1 (USB receiver + DAC + amp) >HD650 (for all recordings)
8) FiiO X5 Coaxial Out > HA-1 (DAC + amp) > HD650 (for all recordings)
And if I still had the HD600...
9) Win7 Laptop > Foobar2000 > HA-1 (USB receiver + DAC + amp) >HD600 (for all recordings)
10) FiiO X5 Coaxial Out > HA-1 (DAC + amp) > HD600 (for all recordings)
This list of chains might be hard to digest, but let me point out that, for my tastes (subjective), using the HA-1, as a self-contained USB receiver + DAC + amp, works best with the LCD-2 rev. 1, PM-1, HD650, and HD600 (of the headphones with which I've spent a lot of time.) These headphones allow me to make use of all of the HA-1's features, including remote control of Foobar2000 and the ability to play any file format, including DSD, and PCM sample rates higher then 96kHz - with my entire library, good recordings and not so good.
Using the FiiO X5 Coaxial Out as a transport is redundant to using the laptop with the HA-1's USB receiver - only for these headphones, which aren't highly resolving, but the X5 Coaxial Out allows me to "escape" staring at a laptop screen - and the FiiO X5 supports all the same formats, including DSD, albeit with a buggy UI, which is improving slowly, via firmware updates.
For only my better to best recordings, I very much enjoy the HD800. My best chains for the HD800 would be at the top of the list, but for the fact that a good portion of my library isn't of sufficient quality to satisfy the resolution of the HD800. Note that the chains I've listed for the HD800 do not include use of the HA-1's ESS9018 DAC section, which is just too neutral (almost cold) for the HD800's signature. To compensate the HD800's idiosyncrasies, I'm using warm DACs, my favorite of which is the FiiO X5's PCM1792A Line Out, followed by the Beresford Bushmaster MkII Line Out.
The HA-1's spectacularly transparent amp section, delivering a whopping 1084 mW (est.) into 300 Ohms (balanced out), does things for the HD800 that I haven't heard with any other amp (keep in mind that my experience is very limited) - including, and I don't think I'm imagining this, a lifting of bass energy. I'm saying that the HD800 really appreciates the awesome power of the HA-1, but the HD800 cannot tolerate the HA-1's very neutral ESS9018 DAC implementation (for my tastes). That's what the FiiO X5 Line Out does so well - bringing down the highs, adding body and warmth, with all the detail you could possibly want. The Beresford Bushmaster MkII is almost as good as the FiiO X5 Line Out in this role, if I want to use my laptop as a source, but the Bushmaster MkII is limited to 96/24 sources (as is the Teradak Teralink X2 USB-to-S/PDIF converter that feeds it).
I'll add that I have the Resonessence Concero, but as a USB receiver and ESS9023 DAC, it's redundant to the HA-1's ESS9018 DAC, although I believe the Concero's USB receiver is ever so slightly superior (for detail) when serving as a USB-to-S/PDIF converter (Coaxial Out).
Lastly, if I had ranked the Centrance DACmini CX (with 1-Ohm output impedance mod) along with these OPPO HA-1 chains, it would follow numbers 2, 6, 8 and 10 (using its USB receiver, DAC, and amp sections) - with the LCD-2, PM-1, HD650, or HD600. For the LCD-2 rev.1, the DACmini CX comes amazingly close to the performance of the first and second chains in my list above. The additional power of the HA-1 amp (1.8W into 50 Ohms) adds speed and control, with improved dynamics. For the PM-1, HD650 and HD600, the DACmini CX lacks nothing vs. the HA-1 in sound quality (for my tastes) - as it has plenty of power for these phones, but the HA-1 completely overwhelms the DACmini CX for features (at a higher price, of course).
Mike