Digital coaxial audio cable suggestions.

Listen to Al. Either go very short to avoid reflections on impedance boundaries or go 1.5m with typical transport transition time that is about 25ns. Assuming threshold in the half of that reflection from the end of the cable (started by the beginning of the transition) has to come later than 12.5ns. Assuming 2/3 of the speed of light signal travels thru 1.5m forth and back (total of 3m) in 15ns just missing threshold point. 1m cable would be much worse bringing reflection in about 10ns - just before signal crossing the threshold. Any disturbance of transition at the threshold will convert to time jitter (equivalent to noise in frequency domain).

Digital cable behaves like transmission line (reflections) when one way propagation time to point of first possible reflection (end of cable) is shorter than 1/8 of transition time (rule of thumb) - about 3ns in our case making for 0.6m. 0.5m cable should be safe but I would go no longer than 12" (as Al stated) since driver is in few inch distance from the connector.

Mike60, Get 1.5m Reflections come from change in characteristic impedance that for higher frequencies can be defined as SQRT(L/C) and depends on geometry of the cable or connector (for given dielectric type). When you have absolutely perfect cables there would be no reflections but in practice it is almost impossible. Reflections contaminate straight transition edge of digital signal changing time when threshold is crossed (level recognized) making signal to jitter in time. This jitter is a form of modulation and as such makes two sidebands in frequency domain. These sidebands are very small but audible because not harmonically related to root frequency. With complex signal containing many frequencies (music) it creates many sidebands that together become hash/noise. This noise amplitude is proportional to signal amplitude and is zero when music is not playing - difficult to detect. It appears as lack of clarity and affects even imaging. Some of this jitter comes from noise (ambient or system) and is correlated but some is random. Using impedance matched and well shielded cables plus providing clean power reduces jitter effects.
Noisy system modulates amplitude of transmitted signal but also makes threshold (level recognition point) on receiver side not steady causing jitter.

It is very difficult to predict how reflections affect the signal because reflected signal bounces back and forth like echo between walls but there are ways of predicting how signal will get modified (Bergerone Diagrams). Read more here: http://pages.infinit.net/alcor/docs/math/TRANSMISSION%20LINE%20EFFECTS%20%20version%20L.pdf

Flashunlock, I'm not familiar with Esoteric separate clock signal but I would clock the source from the DAC, using buffered clock, on opposite edges to active clock edge (source placing bit of data on falling edge and DAC clocking it on rising edge). It is typical synchronous transmission (vs asynchronous S/Pdif) less susceptible to cable jitter but still susceptible to receiver (DAC) system noise induced jitter. I assume that Esoteric has something like that.

Al is absolutely right stressing importance of keeping cables short. Non-magnetic shield does not stop EMI (for instance radio waves) but induced noise travels on the outside of the cable (shield) to ground because of the skin effect. Skin effect does not work at lower frequency EMI but cable is not long enough to become antenna (1/10 of wavelength) unless you make it longer than necessary. In addition, since induced noise returns thru the shield it causes voltage drops visible as signal when shield is used as one conductor (S/Pdif). Longer shield means higher impedance and bigger voltage drops.