I've been well aware of the bass roll-off for sometime but I would like to see something solid, possibly the review you were reading regarding the output impedance, 26ohms is rather high and I'm sure I would hear at least bass or treble alterations especially with sensitive IEM's like TF10, RDB+ V1, or even the Heir 3Ai I have here on loan atm but I do not detect anything bare the roll off seen above and take Skamp's opinion regarding the impedance at least much more over yours.
Regarding the player, I listen with my ears not with graphs, and C3 stacks up better than any ipod, Cowon or Sansa player I have heard, I learn from hands on experience which you can see again the list of sources on my profile. I'm not sure what IEM's you tried but over my time have created some very nice synergy with C3. I am not Colorfly fanboy but recognize decent sound quality when I hear it. The player is not suitable for higher impedance gear anyway as it only puts out 12 mW if you weren't aware.
Pretty much from an Australian point of view, I see these kinds of conversations through and through here on Head-Fi daily, the graphs, the flat curves, the never ending debate between right vs wrong and I'm not interested sorry.
There's a whole thread (actually been two) that add up to over 150 pages of people very happy with Colorfly C3 some of them also reputable members than myself, and the majority of them using low impedance IEM. I tend to take their opinions and add that to my own experience of the player, not some random member who's been around a few months, telling me how it sounds.
Cheers,