I’m not an expert on IEMs, but to add a bit more to the above… If you hear hiss or noise in an IEM or other lower impedance headphone, chances are it’s because the headphone is simply too loud (sensitive in audiophile jargon) for the amount of gain or noise in the amp.
If you have to set the volume on your amp to a low setting for comfortable listening, that’s a pretty sure sign the amp has too much gain for the headphones, or the headphones are too sensitive/loud for the amp. (It works either way.)
To fix this, you can either try a headphone that is quieter/lower in sensitivity (and frequently also higher in impedance). Or try using a lower gain setting on the amp, if it has that option. Either one of these could reduce the audibility of the amp’s noise floor in the headphones.
There are other ways of addressing the issue (as Polygonhell alludes to above). You could get a better amp with a lower noise floor, for example. Or simply try omitting the amp altogether, and using the headphone output on your player instead, if it has one.
Another less likely possibility is that the noise or hiss is originating in your source or DAC. Generally speaking, you want the gain controls as high as possible on these devices, without clipping the signal, to keep the noise floors on them as low as possible.
Finding a good synergy between an amp and headphone (or other transducers) can be a bit of a challenge when you first get into audio. Much of it falls within the general rubric of proper gain staging and impedance bridging though.
The bottom line is that headphones and other audio gear are generally designed with specific uses or applications in mind. And if you try to go too far outside of the general scope or parameters of those applications, the performance is likely to degrade. The degradation in performance can take a variety of forms, such as increased noise, distortion, or clipping, insufficient volume, or too much volume and not enough sensitivity in your amp’s volume control.
There are a couple different methods for comparing a headphone’s sensitivity or efficiency btw. And to compare apples to apples, you need to know which one a manufacturer is using, either dB/V or dB/mW. dB/V will almost always be the higher of the two.