If the impedance of the IEM is not constant with frequency, you'll want an amp with an output impedance much less than the IEM's lowest impedance. Otherwise, each amp will impart a different sound "signature" to the IEM since the amp impedance will interact with the IEM impedance to emphasize and de-emphasize certain frequencies. For example, the SE846s sound warmer with the iPhone 6+ output than with the Fiio X5ii since the SE846s have lower impedance in the higher frequencies than in the bass. The iPhone 6+ has about a 3 ohm output compared to the Fiio X5ii's <0.1 ohm. This interacts with the SE846s ~8 ohms at 1k, but ~24 ohms at 100 Hz (from memory). The Fiio output makes the SE846s sound brighter. However, when using other, higher impedance IEMs, such as the W3 or UM3X where the impedance curves flips (lower impedance for the bass vs. high), the Fiio X5 makes those IEMs sound darker.
To get a flat impedance response, you almost have to have a single driver without a crossover. With a crossover, you want the lowest impedance getting to the IEM (cables, connectors, amp output impedance) that you can get.
I got the Fiio X5ii specifically for the SE846s to help open them up (white filter, olive tips).