I know it been a few weeks since you posted this, are you still having the issue?
When you say you plug the earphones directly into their box I assume you mean plugging
directly into the ME1 output.
With the direct connection earphones sounding good that all but points to an issue between the ME1 and the wireless IEM.
What model IEM are you using and is the ME1 the only thing connected to the wireless
transmitter?
How are you connecting the ME1 to the IEM transmitter, the ME1 mono out or the headphone jack, and what kind of cable configuration are you using?