Grm 5-6-2016
I guess I’m in over my head here, not for lack of trying... Lesson learned!
No, you’re not in over your head at all, IMO. In fact your post struck me as as a very credible recitation of your experience, and as raising some particularly intelligent questions.
Regarding "it’s a digital signal, should they not sound the same?," the reasons differences in digital cables can significantly affect the sonics of a system are technically explainable, and are well established. They relate primarily to small short-term random or pseudo-random fluctuations in the times at which the component receiving the signal detects the transitions that the signal makes between its two voltage states. That is referred to as "jitter," and will generally result to some degree in fluctuations in the timing of D/A conversion.
Among different coax or other electrical cables, the amount of jitter that will result with a given cable in a given system, at the point where D/A conversion is performed within the DAC (which is where it matters) depends on a complex (and mostly unpredictable) set of relationships and interactions between the parameters of the cable, including length, impedance accuracy, shielding effectiveness, shield resistance, propagation velocity, bandwidth, etc., and the technical characteristics of the components it is connecting, including signal risetimes and falltimes, impedance accuracy, jitter rejection capability, ground loop susceptibility, etc.
And with respect to optical cables, as you reported finding those can be beneficial due to their immunity to ground loop issues (which can significantly affect jitter, btw, as well as causing hum), but they also bring additional variables and unknowns into the mix, including the quality and various technical characteristics of the optical transducers in the two connected components.
For more than you’ll ever want to know about the subject, see the following paper by a distinguished academician and researcher:
http://www.scalatech.co.uk/papers/dunn_hawksford_1992.pdfNote, though, that this paper was written in 1992, and arguably the issues it raises about jitter are even more critical in many of today’s systems, due to the greater resolution many of today’s systems can be expected to provide, compared to many of those of earlier times.
Good luck, and welcome to Audiogon! Not all threads here are as controversial as this one :-)
-- Al