[amplifier response driving JH13Pro]Shigzeo's post @ Hydrogenaudio.org further proves my point on multi-driver IEMs. As far as JH13Pro is concerned, when driven w/ a high output impedance source, frequencies below 1 kHz attenuates insanely while frequencies above 1 kHz stays quite well, except the sudden plunge @ ~4 kHz. Yes, they are prone to damping issues just like loudspeakers! Thus if you want total-control on multi-driver IEMs, avoid source devices with output impedance higher than 1 ohm.
[amplifier response of iRiver H120 driving ER-4P with added serial resistance]Well, if you do not mind underdamping IEMs with some skews in the frequency response, you can raise the voltage-to-current ratio by adding a serial resistance, as it is done with ER-4. It will definitely ease the amplifier. Amplifier response goes flatter, and IEMs output distorts further.