A true differential digital output has additional "white noise" imposed over it which is removed at the other end - apologies, I can't find the appropriate link where I read about this.

The debate started in the 80's when CD players became two box units. The advantages gained from the reduced PSU/EMI interference were countered by the jitter incurred between the two units via SPDIF.

The latest thinking is the one box transport / DAC but with an external PSU.

With regard to SPDIF alternatives, DPA played with non standard fibre optic connections many years ago, reborn recently by Weiss between the Jason transport and Medea DAC in their reference products.

PS Audio use an i2s feed over an HDMI cable. I am sure there are many other examples but none have been adopted universally.

On the positive side, M finally realised that similar length SL runs to individual speakers was technically superior to running a cable to one speaker and then daisy chaining through additional interfaces.