Image taken from InnerFidelity.
The AK100II has got an output impedance of ~2 Ohms and the SD2 shows an impedance raise in the (especially upper) treble which causes the SD2 to sound brighter than with a source that has got an output impednce of <1 Ohm.
On the balanced output the AK100II has got an output impedance of ~1 Ohm so you could get a balanced cable for your SD2 or pair it with an amp that has got a low output resistance.
Maybe someone with better physical and mathematical knowledge can calculate how much more dB of treble the SD2 will have when the source has got ~2 Ohms output impedance.
The AK100II has got an output impedance of ~2 Ohms and the SD2 shows an impedance raise in the (especially upper) treble which causes the SD2 to sound brighter than with a source that has got an output impednce of <1 Ohm.
On the balanced output the AK100II has got an output impedance of ~1 Ohm so you could get a balanced cable for your SD2 or pair it with an amp that has got a low output resistance.
Maybe someone with better physical and mathematical knowledge can calculate how much more dB of treble the SD2 will have when the source has got ~2 Ohms output impedance.