FWIW, I tried both an E17K and an E18 using the USB OTG input with my Galaxy Note 5 phone, and both worked fine with no issues. I have not decided a sound preference, but I do like the E17's digital display of bit-rate.
Given 256GB micro SDXC cards, somebody needs to build a very small OTG dongle for phones with 4 connections, one to the phone, one to the headphone amp, and 2 card slots (count these as connections). This could give a very portable, phone based player solution. Most phones just don't have the storage capability for 24Bit audio.