Was looking for a quick and dirty solution to a problem: My Mackie monitors have independent (rather hidden) volume control. So I needed some sort of passive or active preamp to conveniently control them. Basically a volume knob. But then I got the idea of adding a headphone out as well... and maybe even a DAC. On my search I found this option:

http://www.amazon.com/Monoprice-111567-Desktop-Headphone-Amplifier/dp/B00KVVX2QW

It may not be the best option, but certainly looked decent and fairly affordable. It also looked pretty much like the FiiO 9, but with the DAC integrated, and that got me curious and bought it.

Measurements and thoughts on the FiiO 9 are available from NwAvGuy (whatever his name is) here for reference. Whether I agree with his general approach/motivations or not, I'm grateful for his efforts:
http://nwavguy.blogspot.com/2011/05/fiio-e9-headphone-amp.html

Like the FiiO 9, the monoprice has a ~10 ohm output impedance, and a maximum voltage swing of about 7 something Vrms into > 300 ohm loads.

Distortion performance is also very similar to the FiiO 9 (in reference to NwAvGuy numbers):

For THD 1kHz 100 ohms I get about 0.0094% THD.



For THD 1kHz 33 ohms I get 0.0169% THD. Third order harmonics seem to be dominant in my measurements.



SMPTE into 33 ohms is about 0.0486% THD.

Total:



Around 7kHz:



Here is the FR into 33 ohms (+/- 3dB):



Anyhow. All in all, it seems this is indeed an affordable clone of the FiiO 9.

Sound quality wise, I cannot complain given the price and features.