i’m not an expert but i’ve tinkered with iems and have made my own. heres a simplified version of what i’ve learned in order of increasing complexity (take it with a grain of salt):

1. adding resistance in front of the driver, i.e. filters and dampers - these tend to affect high frequencies more. a thicker filter dampens the treble more

2. front and back volumes - allowing air to more freely flow behind the driver allows the driver diaphragm to move. so adding a vent behind the driver can increase bass. you can also add filters in front of the vent hole to fine tune the amount of air you want to let in. adding a vent in front of the driver rolls off the bass, kind of like having a bad seal, but in a controlled manner

3. tube and nozzle dimensions - longer tubes/nozzles shift peaks to the left. larger diameters increase those peaks. diameter also plays a role by affecting resistance—narrower diameters lower the treble. this is why you see bass drivers have long, narrow sound bores

4. multiple drivers - drivers have their own properties and resonance frequencies. you can take advantage of this by combining them and tuning them further using the methods above. you may also use a crossover to divide the frequencies

5. helmholtz resonators and acoustic side branches - for example, the sennheiser ie900, which has helmholtz resonators to absorb certain frequencies. these are side chambers in the sound path. a cool way of thinking about this is considering the properties of the chambers as acoustic analogues to electrical components. the chamber acts as a capacitor, the length and diameter of the channel impact impedance, and dampers act as resistance. by combining and tuning these things, you can approximate the acoustic version of RLC circuits, kind of like a crossover

of course, there’s also DSP and tips, like others have mentioned. i’ll also add that tuning balance armature drivers is more accessible to DIYers since there are fewer things to worry about. with dynamic drivers, there are more factors such as diaphragm material, housing material, cavity shapes and sizes, reflections, etc.