The output impedance of your amplifier affects the frequency response of low-impedance headphones. The rule of thumb: amp output impedance should be less than 1/8 of the headphone impedance. Many cheap amps and dongles have output impedances of 1-10Ω, which can audibly colour low-impedance IEMs.
For reference: Apple dongle ~1Ω, Hidizs S9 Pro ~0.3Ω, Topping NX7 ~0.15Ω, JDS Labs Atom+ ~0.1Ω, Schiit Magni Heresy ~0.07Ω, Schiit Vali 2+ ~6Ω (tube, varies with tube), Bottlehead Crack ~120Ω (designed for 300Ω+ headphones only). Most quality solid-state desktop amps are under 1Ω.