Hi! Not a frequent poster here I must say.
You can explain the difference heard by I don't remember who when using different players.
Many things come into play. First is the quality of the support (CD, DVD, whatever). These expensive audiophile CD are not made of gold just for the cool factor. Gold is more durable, first. It also reflects light better, therefore the laser beam is more certain if it read a hole or a bump.
Speaking of that laser, in a good player il will be more "precise" and the "reflexion receiver" (don't know how to call it) will be error-proof. That's for the player itself.
Now the connexion. I don't know much about optical cables, but in the coaxial one, digital signal is not sent as "some current means 1, no current means 0". Instead, there is a certain defined potential that means: above it 1, below it 0. Of course, we try to send potential values far away from this limit to avoid any confusion, but it is still error-prone, especially if it's not shielded properly or else.
As for what I know of optical cables, they are very fragile physically. If they are bent too much the signal can be lost, and if the internal structure is not perfect, the light beam is seriously wrecked when it cross the imperfection. There are probably also some problems due to uncertainty of the signal like in the coaxial case.
Despite all this, I must say that the laser beam of the player is in my opinion the thing that really matter. The cable is so short compared to all the path the signal went through inside the player, its effect is negligible. And for what few glitches that might appear once in a while, error correction should do the job.
Thanks for listening
