Sensitive IEM, we are talking really sensitive iem that will produce hissing at 1 OI (output impedance) etc, such as Andromeda. Over sensitive IEM has a significant sound different on every different output impedance. Yes, this is not placebo.
There is FR of andromeda with different OI. All of them are different. Significant difference.
Edit: usually ba drivers, that needs less current. Overpowering it will make the bass boom. It depends on the user, love boomy bass or accurate bass. I do love accurate bass.