I measured both the O2+ODAC combo and the Audinst HUD-mx2 using RMAA, with headphones connected in parallel to realistically load the amps. I used EMU0404USB to record the testing signal. I also had measured Schiit Modi+Magni stack a few months ago, then I sold the Schiit stack. My measurements also include the excellent integrated audio of my Gigabyte Z97X-Gaming GT motherboard and the headphone output of the EMU0404USB. I'm not sure if I am allowed to post results of measurements here on Head-Fi, also, I could not stress enough that my measurements are not absolute and precise, as RMAA is not perfect and the EMU0404USB adds some THD, IMD and noise to the results on its input (as well as having the dynamic range limited to ~114 dB). But I can tell you a few things I noticed. Yeah and I also need to list the headphones I used for the measurements: HD650, AH-D2000, DT-770 600 Ohm.
The results indicate the following:
AMPs:
1) The Magni had the best results of all amps with the Denons (low impedance). However, once you turn the volume knob past 12'o clock, the THD raises extremely high and around 2'o clock volume, the Magni was the worst measured amp of all I measured.
2) With the Senns and Beyers (300 and 600 Ohms), O2 was the best, with Audinst close behind and Magni right after that (until I pushed the volume on the Magni, see just above)
3) When doing blind testing with my friends, we could not tell the O2 from the Audinst (when comparing the two amps, we always had to use the Audinst as the source, as the Audinst's amp does not have any external inputs). Nor could we tell them apart during sighted listening comparison
4) To comment on the integrated audio of my motherboard and the headphone output of the EMU:
Both are good, measured great, in some areas better than the rest (dynamic range with high impedance headphones), in some worse (THD, IMD), nothing notable. Both are plagued by high output impedance and do quite bad with the Denons. The integrated audio has more power (volume), but no analog volume control.
DACs:
1) Worst results are Audinst's when using USB input and Modi's. More on that below.
2) ODAC is very good, considering the DAC is powered purely by USB.
3) Of all my DACs, the best results were: the Audinst, when using optical input and right behind him was my EMU0404USB.
To comment: the Modi is not a very good DAC in my opinion, because it uses USB power and does not handle PC noise well. It could be a great DAC if it used external power or if computers did not generate noise. BUT it does significantly better out of a laptop; I however use my desktop PC for listening so that is not an option for me. Also, I can imagine the Schiit Wyrd will help significantly. Also, it inverts the signal on the output.
ODAC does very well, especially considering that it's powered by USB (even when you like me, have the O2+ODAC combo, the ODAC is NOT powered by the wall-wart, but purely from USB). NwAvGuy managed to filter out the USB noise very well. Still, the EMU0404USB, which uses an external power supply for its DAC, measures better, so does the Audinst when using optical input.
Audinst as a DAC....well...very interesting. To be honest, I'm not sure if I heard anything wrong with the Audinst when using USB connection. I think I did, but I'm just not sure. One thing is clear:
The Audinst measures VERY MUCH better when using optical input. In fact, it measures unbelievably well on optical. On USB, the opposite is true - quite bad. THD, noise and IMD near the level of audibility (-80dB). From the graphs that RMAA generates, I would say it's jitter. Not sure. But I'm not connecting my Audinst via USB again, that's for sure.
FYI: I used my motherboard's integrated audio's optical out. When using USB, I had the Audinst powered by the external adaptor. For all USB connections, I used my motherboard's USB ports designated for DAC connections (they should have as little noise as possible).
All in all, I would recommend the Audinst HUD-mx2 over the O2+ODAC combo to anyone, who can use it via optical. It is just great. If you have to use it via USB, chances are that you will not hear its shortcomings, as they are just around what is considered the border of audibility, but I strongly recommend using the optical input.
The O2+ODAC has a very slightly better amp (with lower impedance, too) and has no problem with USB.
Off topic: The Schiit stack is also an option, but with quite some limitations: If you are using a desktop computer, get the Wyrd as well. Modi needs it. With a laptop, you should be good. Also, Magni does not handle volumes past 2'o clock well. Use it with low impedance, sensitive headphones.
I hope this has been informative for you and I'd like to thank you for reading.