I was enjoying my Fiio E7/E9 combo with my Beyer DT990/600s ... and then I tried the Fiio combo with my Denon D2000's.
Long story short, my E9 volume pot is incredibly noisy -- emitting loud scratchy static -- with the Denon's. I emailed Micca Store, where I bought the E7/E9 combo in December 2010, about the noisy E9 pot. I received a pat response that the E9's noisy/scratchy volume pot is a known issue but that it is not a problem because the pot does not make any noise when it is stationary.
I replied that I think the noisy pot is a problem because I often adjust the E9 volume during playback. I asked diplomatically in my follow-up email as to whether Fiio would have a fix or a replacement pot available. This email was ignored by Micca Store. Several days later, I sent a follow-up to my ignored follow-up email and was ignored once again. Several days after that, I politely requested a refund on the E9 if there was not going to be a volume pot fix. Micca Store ignored me for a third time.
In my opinion, the moral of this story is: Fiio's quality control is poor, and Micca Store's customer support is likewise poor. YMMV, but caveat emptor.