I just installed the Nu Audio card, idea being that I can replace a Schiit Modi+Magni stack on my desk to clean up some space.

Looking at the specs, My HIFIMAN HE-560 (v1) aren't nearly as hard to drive as I thought they were -- they're only 45 Ohms impedance -- but even at 100% Master Volume / System Volume, 100% headphone volume isn't very loud -- only a normal listening volume, so I wouldn't be able to turn up things that are hard to hear or play something immersive.

Am I the only one experiencing this issue or might it have something to do with the fact that the cans are planar magnetic?

Reading another thread, it seemed like it's better in terms of audio quality/interference to have the volume around 50 or 75% max, so I'm just concerned that I lose out on audio quality if I have to have them at 100% just to get normal listening volume.

Yeah, I have settings at 100% Master Volume, and 50% headphone now -- I played around some and it seemed like the headphone volume had no difference at 4%, 25%, 50%, and 100%, so I just put it in the middle

What connector are you using? Have tried using a different one?