Just my two cents on digital signal, or SPDIF in particular...
In normal cases, where you spent virtually NOTHING on the digital cable (optical or coax), Coax will DEFINITELY sound better. This is due to the huge diameter of the Toslink fiber optics, where internal reflection with such large diameter of cable will cause some of the light pulse to arrive later, some earlier. instead of getting a sharp on/off, you get a slow brightening/dimming effect (to the detector), and this cause the receiver to trigger at unpredictible interval, and thus creating jitter.
Since clock is part of the SPDIF signal, jitter here will just screw everything up.
Coax on the other hand, does not have this problem, at least not as bad.
However, if you plan on spending on a cable that costs much much more than Raspberry Pi (that's what I'm using now), you can get optical cable the works better than most coax cable, plus you also get optical isolation thrown in for free.
My own experiment is that multi-fibered optical works much better, likely due to smaller diameter of each fiber limiting the path that the light can travel, and thus reducing the effect of some signal arriving earlier/later. (I don't think you can eliminate that unless you go single mode, but I don't think we are going to see specialized single mode fiber here anytime soon.)
The more fiber it have in the same 1mm diameter, the better the performance, but the law of diminishing return works here, you will be paying a lot of money for the last bit of improvement. To some, it is utterly stupid, since it is not their setup's weak link, and often a cheap coax will work just fine for them. To others, who might already be spending a pile of money on their system, it could be the cheapest upgrade in their system, which would work out well for them.
BTW, just setup my Raspberry Pi 2 + Digi+ compatible digital output a few days ago... and damn it sounds great... I'll check back and see which sounds best, I currently have 3 ways to connect to my DAC:
Optical
Coax/BNC
USB (This one can do DSD!! yes!)
It should be fun trying them all out. To anyone who want to test different interfaces, quickly listening to a short piece and switch quickly to another usually don't work for me. I usually listen for a while on one, then switch to another for a while, this is where I can pick up the differences.
