Immediate reaction: The ES100 is magic. LDAC connectivity so no worry about degradation with Spotify streaming. Sounds effing fantastic, clipped the unit (which is WAYYY tinier than I expected) to my jeans, good to go. This 100% has my endorsement right now. I agree with others who called the Dragonfly slightly "clinical" in comparison, the low end feels more potent but not by a huge margin. The ZSX is just big and FUN.
EDIT: I just feel the need to repeat how damn good this little fella is. Particularly for a metalhead like me. Threw on some Lord Mantis, Thou, and a couple other black metal / sludge bands and just... the music with ES100+ZSX is so punchy and lively I'm bad to melting into them. Thank you to everyone for the suggestions. I prefer this to my DFR now.
FURTHER EDIT: I do have one minor qualm. The power on/off sound is stuuuupidly loud. Like when I turned it off I jolted because it was louder than the music. However, it sounds gorgeous as a USB DAC. I will say that the Dragonfly Red is likely "technically superior," I just definitely enjoy the ES100 for jamming music more. I'll also say the app is ABSURDLY robust, I can't imagine messing around in it to its full potential.