To be fair, I was thinking of mod'ing the C3 as well, but it just doesn't have enough space inside for me to do anything about it. I don't mod things just because I don't like them. I mod them because I believe I can improve them (*it is a common trait for most DIY'er, constantly looking for way to mess around).
But that aside, don't take the issue to one side. Yes, C3 offers better clarity and upper end detail. But in contrast, X3 offers better bass hit, speed and lower-end texture / detail too. What I have said is that if you prefer clarity over other aspects of the sound, you will probably prefer C3. But sound is more than just clarity, and of course we all have to decide what is more important to us personally.
As for bugs - I don't think C3 has less bugs in its days. It certainly has its own share of problem but it is also a lot longer in the market for ColorFly to iron out most of the problem and it is a much simplier device to begin with. In comparison, FiiO has been much more active in firmware development than any other Chinese DAP maker that I know of. Where else does you see a manufacturer releasing bugfix in monthly (and sometime weekly) basis?