Both analog and digital cables made a difference in my system. I was in the cable business 10 years ago or so and I tried to make as many simulations and measurements as possible. I had access to some powerful modeling software, so I modeled power cables, speaker cables and analog interconnects. I now have access to some jitter measurement equipment that allows me to compare S/PDIF coax cables. Learned some interesting things:
1) 6-feet of PVC insulated stranded 14 gauge power cord has the same inductance as 30-feet of ROMEX in the wall.
2) many stranded copper cables have a lot of oxidation on the strands even before the cable is stranded/fabricated
I avoid un-plated copper stranded cable for this reason. Tin or silver-plated is best if you use copper. Solid copper also fine.
3) conductor metals have an effect on sound quality and it is related to the molecular structure/ductility. The more ductile the conductor metal, the better conductor it is for analog and is less likely to degrade with "working". Working is bending the conductors.
Here is a TDT plot showing two identical silver interconnects, one of which was immersed in liquid nitrogen to break-up the crystal lattice. The LCR measurements of each cable was identical before and after the immersion:
View attachment 23895
The red plot is the cable that was immersed in liquid nitrogen. Those will argue that these perturbations are very high frequency, beyond audibility, but believe me, the immersed cable was unlistenable. I threw it away.
As a result, I try to pick OCC pure silver for all of my cables, analog or digital, and they do sound better to me.
4) S/PDIF digital cable can add a little to the jitter or a lot. Depends on the cable. Here are a series of plots comparing various different S/PDIF coax cables:
https://www.audiocircle.com/index.php?topic=154425.0
5) it is wise to make the minimum length on any digital cable about 1.5m. This has to do with the reflections affecting the waveform when it is sampled and the resulting jitter. Here is my white-paper:
http://www.positive-feedback.com/Issue14/spdif.htm
Steve N.