It's been a while, but here are some further measurements and commentary. The following measurements were done at low gain, which is what I usually use when driving my HD600s. Distortion numbers in general are very decent. I chose a 40 kHz BW to accommodate for the 20 kHz THD+N measurement. In terms of voltage available to 300 ohm headphones, consider that 175 mW is equivalent to a fairly respectable 7.24 Vrms. Frequency Response is fairly flat through out the audio range (+/- 0.1 dB). Below is a plot of the 10 Hz to 20 kHz frequency response while using a 33 ohm load on both channels. Volume knob is at 12 o'clock. One of the main issues I have with the SR1 is that it does not seem to allow visualization of THD+N vs Vout. However, below is a THD+N vs Vin sweep using 33 ohm and 300 ohm with both channels running. The cuttoff (1% distortion) for the 300 ohm loads corresponds to 7.24 Vrms, and for the 33 ohm loads corresponds to 4.24 Vrms. One should be very careful when interpreting the distortion numbers. THD+N is referenced to fundamental. For the 300 ohm load, 0.0005% is achieved around an output of 361 mVrms. To observe the distortion behavior across frequency we can plot the distortion vs frequency at some nominal operating value. This is useful because THD+N is typically done @ 1 kHz only. A device might have no issues @ 1 kHz, but it could strain at other frequencies, particularly in the high frequency audio range. For 33 ohms THD+N vs Frequency is characterized @ 200 mW output. While distortion does raise @ higher frequencies, it does seem well controlled. Next we characterize with a 300 ohm load @ 50 mW output, with similar conclusions. Next we see distortion numbers when the device is driven by 40 linearly spaced tones in the audio range (about 1 Vrms output using the more demanding 33 ohm loads). As can be shown, THD+N is -70 dB, THD is -82 dB and IMD is -75 dB. This is a HiBW measurement (200 kHz).