I did a quick assessment of the amp using RMAA (for what little that's worth) and my cheapo USB X-Fi Surround 5.1 sound card. Measurements are limited by the line in on the sound card in many cases. This is with default Windows drivers, so 24-bit modes don't work; I tested 16-bit 48 kHz only. For some reason, the line in level is not calibrated right using default Windows drivers somehow, so the noise floor is a bit high I think.
First test was sound card loopback (no load). Then I tested with three different headphones in turn: AKG K601 (120 ohms nominal), Beyerdynamic DT770 Pro (80 ohms nominal), Beyerdynamic DT 235 (32 ohms nominal). After that, I tested with the sound card into the O2 (no load), and then into the O2 with those same three headphones.
My O2 has gains of 1x and 5.1x. Below tests are for 1x gain, on AC power. I started to test the 5.1x gain setting, but the results were all the same, so I got bored and didn't finish that. I also spot checked with battery power only, but that looked the same too. THD = total harmonic distortion; IMD = intermodulation distortion (SMPTE test). Though really, these are looking at THD + noise, and IMD + noise.
All tests were at the max line out voltage of 1.1V rms, measured with a multimeter. Actually, the sound card's output impedance is nontrivial, so the headphones were getting a little less than that on some tests, but not with the O2. Click on graphs for large versions as usual.
Sound card (no O2) THD:
0.0027%, 0.018%, 0.025%, 0.046% (top to bottom, meaning unloaded, K601, DT770 Pro, DT235).
Sound card (no O2) IMD:
0.0098%, 0.025%, 0.096%, 0.050% (top to bottom).
Sound card -> O2 THD:
0.0028%, 0.0026%, 0.0025%, 0.0025% (top to bottom).
Sound card -> O2 IMD:
0.010%, 0.010%, 0.010%, 0.010% (top to bottom).
Again, note that results are limited by the sound card. Also this is just RMAA. Don't take too seriously.