It is reported by Radio frequency Engineers that SPDIF cables needs to be 2 meters or longer on a properly terminated 75 ohm line, shorter than this leads to all sorts of cable reflection problems. These reflections will be heard! This could account for some of the perceived differences between cables.
Edit: this is not a recommendation for buying expensive cables - just simple engineering & no magic ingredients!!
Edit Again: I should have also said that making the cable as short as possible will also work with non-ideal SPDIF (RCA connections etc.) - 12" is possible short enough
You said the reason for keeping the length at least 2 feet had to do with reflections. Reflections have to do with MORE LENGTH, not with less length! Reflection becomes an issue when the cable becomes LONG, making the signal propagation delay longer (the signal travel time from the “driver” end of the cable to the destination “end”). What does longer time mean? Longer with respect to the digital signal rise (and fall) time.
A typical cable delay is around 1.5 nano second (nsec) per foot. The velocity is slower then the speed of light, in the range of 1/3 to 2/3 of the speed of light, and it depends almost entirely on one factor - the cable inner material isolation (the dielectric).
The rise time for the digital signal is between 5nsec and 30 nsec. 30nsec is slow but still within the specifications. 5-15 nsec is nice, and the reason that faster is not allowed has to do with setting a limit on the electromagnetic radiation (transmission of interference).
At say 10 feet, the cable delay is around 15 nsec, and a 5nsec rise time is 3 times faster then the delay, so one DOES NEED to terminate the cable and do so properly.
But at say 8 inches length, the delay is around 1nsec and even a fast 5nsec rise is 5 times slower then the cable delay, and the signal will have virtually no reflections at all. The shorter the cable, the better it is from reflections stand point as well as from many other standpoints.
I am not suggesting 8 inch cables. I am not suggesting not terminating. In fact, as a rule the termination is built into the receiver side. The issue here is cable length, and the notion that there is a minimal cable length one should keep is just plain wrong.