I'm a big fan of the zO2, but I listen at normal levels. Every amp will hit a limit when reaching huge volumes with certain headphones. The zO2 is an external EQ and amp and it does both very well indeed. At normal listening levels the Pro 900 will NOT distort like that. The amp circuitry of the zO2 isn't as robust for high volumes, but then again, who listens at over 95dB? Or, if you do, why?
Just sounds like you are overloading the input on the zO2. Back in my concert taping days, I had to get an external low pass filter for live recordings as the bass from concert subs would sizzle in the mics. The Fiio may have a more robust input system and that is it. Output, the zO2 is hard to fault. But then again, the 24dB of output from rock box on a clip leaves something to desire.
Edited by shigzeo - 5/2/12 at 6:29pm