The other thing to think about with amps is the output impedance. FiiO amps generally have a very low output impedance (less than 1 ohm), which is very useful if you (a) have a source that has a high output impedance and (b) have a headphone that has a highly-varying impedance curve and goes to a low value (say around 10 ohms) at some frequencies. Many IEMs have a highly-varying, sometimes low source impedance, and something like a FiiO amp tames any frequency response variations due to impedance mismatch.
Having said all that, the M100 has a very flat impedance right around 35 ohms, so pretty much any DAP with 5 ohm or lower output impedance is fine. That includes pretty much all iDevices, all Sony DAPs, and several of the higher-end players, too. Here's a link to an output impedance compilation (Zo column):
Innerfidelity's data shows the impedance curve for the M-100.