A balanced cable keeps the signals from the left and right channels completely separate. An unbalanced cable has a common ground wire. This can lead to noise and crosstalk. AK240 has separate DACs for each channel. An unbalanced cable with a shared common wire might defeat or partially defeat the purpose of having 2 separate DACs.
Thank you, I think I've just got it :-)
A normal single ended cable has Ground and signal - i.e. a direct potential difference between the ground and the positive line controls the volume you hear. In the 3.5mm headphone out of single ended amps you do have a common ground and the left and right singals on separate wires.
In a 4-pin balanced output, like the 2.5mm TRRS from the AK240, there is no ground atall, instead there is an additional inverted signal for both sides. This increases (doubles really) the potential difference between the signal and inverted signal, but also removes any interference that has the same sign on both lines. Hah, simple and cool.