High sensitivity headphones are great because they don’t require a lot power to drive them but the downside to sensitive headphones is that they tend to produce audible background hiss that transfers into the audio you’re playing. How much hiss you get depends on the audio source the headphone is connected to (i.e. smartphone, digital audio player, single-ended triode amplifier, etc).
One way to solve the background hissing (or humming) you get with high sensitivity headphones is by increasing their output impedance, which works great with single driver IEMs but not so good with IEMs that have multiple balanced armature (BA) drivers because each driver has different impedance so, if the output impedance of the audio source is higher than zero, you can get a change in frequency response (i.e. lower bass response).
The easier way to solve background hiss and imprecise control of volume is with an attenuator like Ear Buddy and iEMatch, which are audio attenuators that reduce the output of a headphone amplifier to minimize channel imbalances at low volume and background hiss…