The issue with differing resistances in cables is that most IEMs impedance curves aren't linear so they have a reactive change to different resistance or impedance of a device or in this case, a cable.
Yes, but it's about relative level of that resistance that effects the output of the headphone, and this is in relation to impedance response of the headphone.
Planars should not be effected as it's a resistive load. Same goes for dynamic driver iems out there that are nearly flat, but for BA impedance responses that are nonlinear with impedance level that dips closer to cable impedance, depending on the relative impedance in comparison.
Some BA iems have very low impedance dips, and those should have audible difference depending on cable impedance due to relative differences that are significant enough. Bartsky's measurements illustrates this with Andromeda which can dip low in impedance response.
Look below on the Adro's impedance response. The maxima is quite high, but the minima at around 3ohm is very very low. So, bass get's effected, but the treble not really, mids somewhat.
From Headflux:
"Likewise, the Inear responds to the output impedance of the source device. The above figure shows how the Andromeda reacts to different output impedances. The upper yellow line shows the Inear at a 0 Ohm output, which accordingly does not cause unwanted deviations of the frequency response. Below are outputs with 2, 4, 8, 16, 32 and 64 Ohm. So it can be seen that the Andromeda sounds with increasing output impedance of the player, amplifier etc. increasingly brighter. Our recommendation is a playback device with an output impedance of max. 0.5 Ohm - higher values cause changes in the frequency response of more than 1 dB, which can significantly alter the sound of the Inears."
Now if you look at HD800's impedance response, you see that it's quite significant compared to 3ohm low dip on the Andromeda as the lowest is quite high at 350ohms. It's going to take much greater impedance to effect the HD800. So, the point is, it's the relative impedance levels of the headphone compared to cable impedance.