Yes, I think you're right about the iem's. Not because of sensitivity but because of their impedance. Re Nx3s, starts rising around 4V, graph show 0.0005% at 5V, which is still very good, if the graph is to believed. More to the point, why a portable amp if you are going to plug into D50? Desktop would be a whole other conversation. Amir says JDS Atom almost looks like it is a brother to the D50, and that is, without question, the desktop pairing to get.
I basically have a quad stack on the train chair next to me.
USB Battery Pack
Might switch the D50 + Battery to a Khadas Tone Board if I can print a good enclosure for it.