Received my C5D in the mail today. I tested it with my Mad Dog headphones and my RE-400 earphones. Everything is great with the Mad Dogs -- no hum present. It is noticeable with the RE-400's, though -- enough so that it could make listening to music with soft passages problematic for those with more sensitive IEMs (or for those who are particularly picky). Granted, the RE-400's are sensitive enough that there is a noticeable jump between having the volume all the way down (or silent, in other words) and having it one step up from that, so I'm not sure how representative my experience is.
That said, the hum doesn't appear to change with volume, so I think tvolpe1600's observations may be something else. Also, what I'm hearing only lasts for about a second or two once audio output from the computer has stopped, which goes along with jseaber's note that it stops when the DAC is paused. Finally, what I'm hearing is not present when using it as an amp.
So, then...I guess the question is: am I going to be sending mine back? I'm still undecided, but am honestly leaning toward "no" right now. The reason is that I bought these for my Mad Dog, which needs amplification to get as loud as I like and sound its best. I typically don't listen to headphones with an amp unless the pair I'm using really needs the extra power.
In any case, major kudos to the folks at JDS Labs for how they've handled this. Your open communication and commitment to excellence puts many of your competitors to shame!