Its all ones and zeroes isn't it?
One of the most puzzling things to any audio enthusiast is how digital equipment can sound different as its all ones and zeroes. For CD players, the apologist could say that the digital part sounds the same, however, better CD players have better quality analogue outputs. Alternatively, they could say that the particular digital filter used by the CD player acts as a sort of 'graphic equaliser' (witness the Sony CD players with switchable digital filter settings).
However, I would like to say that this 'switchable digital filter' thing is a bit overrated. I remember SAVE '97 where the Sony display was devoid of people and one audio enthusiast and I actually sat down and started switching around the filter settings and listening to the differences. I would say that the difference is subtle (though it was a show, the listening conditions were good, mind you) But yes, the digital filter can make a difference.
My take on the digital stream from CDs is to remind people that the data for left/right comes out in a series (and not simultaneously) and after that they have to be 'reassembled' to play simultaneously. Its a tough job, that. But note, the analogue domain is involved.
But how does one explain differences where everything remains in the digital domain? Personally, its a mystery to me. If you really want to know more, just post a question on rec.audio.opinion and watch the flame war begin :)
As I've always done, I prefer to just tell you what I hear. If you don't hear the difference, don't waste your money, buy more CDs instead :)
JVC Toslink (optical) digital cable vs Audioquest Video 2 75 Ohm co-axial digital cable
Using my Sony EP9ES surround processor as a DAC and a Panasonic DVD player as a transport, I compared the 2 cables. There is a difference... just barely significant enough for me to be confident of detecting it A/B (which I did anyway). There is a little bit more 'space' in between things like female vocals which made the co-axial cable superior. This sort of difference is actually more apparent if you go very close to the speaker and listen carefully :)
My listening abilities stop here though. I can't hear the difference between different co-axial 75 Ohm cables, though others can (but note, I have a crappy DAC - its not even a DAC, its a surround processor :)) So my only criteria for a digital cable is that it must be a true 75 Ohm cable (check carefully - it doesn't matter if its labelled 'for Video' - theres no difference) and it must have good connectors (connectors are very important, also, I have to plug/unplug frequently when swapping between LD/DVD). Silver plated OFC is supposed to be the best as it maximises bandwidth but whether it translates into audible differences is another question.
Panasonic DVD player vs Sony XA7ES as transport via co-axial cable to EP9ES as DAC
The difference was truly amazing. With the Panasonic as transport, the there were colourations to the midrange which cheap hi-fi usually has to make things sound more exciting, but really, its just rough and brash. Timing was poor and the music often sounded disjointed, even incoherent. The soundstage was flat and 2 dimensional as well.
With the XA7ES as transport, I couldn't believe that the DAC was still the same. It sounded like I've moved up to a higher class of CD player altogether. The sound became, smoother and more refined and the pace and rhythm improved. Soundstaging also improved.
I am truly, truly impressed. I had my doubts as to how important a transport was but this has dispelled any such doubts. Oh yes, the XA7ES still sounds better by itself of course, but others have reported using the XA7ES with high-end DACs with good results.
The CD itself
I've covered the topic elsewhere, but just to summarise: yes, different pressings of identical CDs can sound different; ergo, CD-R copies of CDs can sound different from the original.