Thanks for everyone's help earlier. I received a replacement and the new one is much less wonky with the display noise. I feel I can still pick up when the amp circuits are active, and when they cut out with lack of activity, but I suppose I am kind of sensitive to these things and also have not listened to headphone dac/amps in a while to remember how much of it was generally there. That and perhaps the Cardas balanced-cabled IEM sensitivity is such that on this issue it is not the best match or needs a lower gain setting, I barely get the volume above 1/4 way usually although I don't listen loud. This slight amp noise is not affected by volume, so I am not thinking it is anything terribly weird at this point, just an active circuit. Cardas offered to test the cable and headphones if I wanted but I will wait a bit to decide.
The Cardas EM5813 headphones are a great match when the music is playing though, really fantastic so far. Through this process I switched from the balanced mode to my Etymotic ER4S to try and figure out what is going on and I was pretty shocked how they paled in comparison. Recognizing that is a difficult headphone to drive I tried my Etymotic hf5 that gets used right out of the iPhone, and out of the Pono it was not leaps and bounds better than the iPhone; maybe over time the differences would become more apparent however, as I was more quick-testing, but then when going back to balanced, wow it just hits you big time, another world really and no doubt far closer to being in front of my main stereo.
Beyond that I cannot really add much as I don't have a ton of experience with a variety of headphones or dac/amps and this is my first DAP and first balanced headphone. I know it is early to be making big judgements, but I will say I have been plenty disappointed right out the gate with a new DAC/AMP amp, the HP-P1, which cost more than this player, so I feel confident this is not a case of spending money and forcing yourself to like/justify it. I am not going to win any Pono Scroller of the Year award, but I look forward to using this a ton, and maybe seeing what some other balanced IEMs can do searching for a great match.