Looks like it outputs 67mV into 8 ohms. That's only 0.595mW. Not nearly enough to drive headphones like the HE-500's, but those numbers seem suspect. 0.595mW isn't enough to drive many headphones at all. Especially since even the lowest impedance IEM's are only 16 ohms (lets say...), which means that those tiny IEM's are only getting 0.2975mW.
If a very efficient IEM were to be used with that unit:
dBSPL = Sensitivity in dB/mW + 10 * LOG ( Pmax in mW) = 118 (JH16 Pro) + 10 * LOG (0.2975) = 105dB (just barely loud enough to power a tiny IEM).
I don't like that spec of 67mV into 8 ohms. It seems way too low? Surely it would be higher than that.