I use pure 24k gold USB cables - it makes my reference HP laser printer print much sharper with increased clarity and resolution, and much improved depth in the blacks!
Thats not a like for like comparison though, Printers use a buffer, and are not a real time application, so if an error is detected in the datastream it can be retransmitted. Also jitter is meaningless as the data will be stored in a buffer in the printer before its printed.
On the other hand, SP-DIF doesnt allow retransmission, so if there is a bit error, error correction will make a "best guess" at what it should have been and pass it along to the DAC anyway. As DAC's are normally unbuffered, jitter (small errors in digital clock signals) will also make audible differences in the final sound.
Take the following "visual example"
1 1 0 1 0 1 0 0 0 1 1 1 0 1 1 0
1 1 0 10 1 0 0 01 1 1 0 1 1 0
To a computer, or a laser printer, the end results are exactly the same as the bits will be recieved by a buffer which means only the bit values are important, but the timings are meaningless.
However to a DAC running in real time, the timings can and do affect the sound which is output from the dac. As I already mentioned before the other issue is SP-DIF, and HDMI are real time streaming interfaces which use error correction instead of retransmission. While the correction is pretty good, and you will be able to listen to music even with a fairly high error rate, the fact is your not actually listening to a "bit perfect" end result, mearly a close approximation.
On the whole, copper cables are better for short runs, as there is no need to make the conversion from electrical to light and back again, this removes one possible source of jitter. However on longer cables, optical does have the advantage of far lower transmission losses.
With your denon system, I would stick with a short coaxial cable. It should have lower jitter, and on a short cable bit errors will be negligable.
