Yes, I know the impedance switching and their order..
It caused some confusion for some..
Okay, here we go... I believe that increasing the impedance actually benefits the N8 universal, due to crossovers and impedance swing, it actually controls the their upper/mid bass level better and thereby gave more room for the midrange and not sacrificing the upper treble.
The gain at level 1 actually brings the M8 to it's real line out level, which is also conformed by Centrance to use gain 1 at near max for line out analog level when feeded into another amp.
Of course I don't listen to IEM"s a that level unless I want to loose my hearing in a few minutes
but the gain 1 seems like what I have experienced to be the optimal setting, even with the N6 and maybe other IEM's. I alsoI prefer my Ety 4ERs as well at this setting.
I strongly suspect that any given IEM with multiple BA crossovers will behave differently so no general rule to be set, just that the M8 seems very versatile and maybe can accommodate to a variety of IEM's due to the M8's many combinations of little hardware implanted switches....81 possibilities/options, right?