I wouldn't agree with slim.a on the post for noise floor, the C-2 is not the quietest I've heard,with my Grado 325is even with no DAC connected the complete darkness disappears at about 2'o clock and same on high gain, I do have cx300 will test and advise later.
The reason you start hearing the noise when you turn the volume so high is that the C2 can output as much as 10v. (See the specs here: ÐÂ½¨ÍøÒ³ 1).
The reason you don't hear the noise floor with something like the emu0404 usb for example is that it has very limited output. The headphone output is limited to around 2v.
The C2 was made so that it could drive difficult cans like the HD650 and K701 with ease and a lot of headroom. That is why I suggested to SoupRKnowva to ask Kingwa for a modified version (if he accepts doing so). By lowering the gain/maximum output, it will be probably possible to lower even more the noise floor but it will cause it to lower the headroom with low sensitivity cans.
BTW, kunalraiker, how high do you turn up the volume with the Grados? Even with the HD-650, I rarely go past 2 oclock ...