You are welcome!
Most portables will have "unity gain" in place. And jamato8 is correct here. And there is very good reason that it is the "norm" (especially for portables).
I think you have the right of it. The lower gains are mainly for sensitive IEMs and having finer control over your overall volume.
So, the technical definition of the concept is this: A
unity gain buffer (also called a unity-gain amplifier) is an op-amp circuit which has a voltage gain of 1. What this means is that the op amp does not provide any amplification to the signal. The reason it is called a unity gain buffer (or amplifier) is because the output voltage signal is the same as the input voltage (hence the name unity). This is especially important on portable devices that are not capable of making a ton of current (being battery powered) because an op amp circuit is a circuit with a very high input impedance. This high input impedance is the reason unity gain buffers are used. When a circuit has a very high input impedance, very little current is drawn from the circuit. Bottom line is that the Unity circuit draws very little current, not disturbing the original circuit, and giving the same voltage input signal as the output. They also act as isolation buffers, isolating a circuit so that the power of a circuit is disturbed very little as well.
Some music might be more noticeable than others where your hearing, and noticing it, is concerned. So, YMMV and it might be more desirable to listen in medium or even low gain even knowing you aren't getting "all" the potential sound you could be getting; just like a tube amp that adds distortion to the sound.
No, I believe that the normal signal is still high gain on the Amp12 card. But remember that this card is capable of over 2W and 2Amperes of current, which could both damage your IEMs and/or your ears easily with higher dB of volume.