The "unofficial" line input on the Red Label (Diablo) is single ended, as the whole headphone amplifier is single-ended input and actually a single ended Headphone Amplifier that then creates a the balanced output, like so:
It was a solution dictated by basing the Red Label directly on the iDSD micro and just "bolting on" balanced as an afterthought because marketing demanded "must be balanced" when the development had already gone to prototype level.
I did suggest a thorough ground-up redesign to be differential DAC to Headphone (which would also make the line in balanced) and rolling in the advances in usability and performance in some areas realised in the xDSD/xCAN, but was instead told to just rush the implementation of making the iDSD micro "balanced" and to somehow add "balanced". I did that as well as I could, given the design brief. I would not consider the best I could have done for this kind of device, but the best I could do within the limitations imposed on me.
Incidentally, this cable:
Will not only provide a SE Input to the Red Label, but also an output.
Thor