mike'n'ike has an excellent point. My experience until very recently has been similar -- I haven't been able to find an audible difference between most digital cables, both optical and coaxial (Monster Lightspeed 100, Monster Datalink 100, Bettercables coax and optical). And, if memory serves correctly, coax digital cables are required to be 75ohm, just like composite video cables, so there really shouldn't be a problem with using a video cable as mike'n'ike describes. (The specs for BetterCables' digital coax and composite video cables are identical.)

I will add one thing. I recently tried a fairly expensive audio interconnect in the role of coaxial digital -- a TMC White Label (I checked with TMC to find out if it was capable of being used as a digital cable, and King at TMC confirmed that it could be). At $80 for a single 1-meter White Label (or $90 for a single 1-meter Yellow Label), it's more expensive than anything mike'n'ike offered, but it did produce a readily noticeable difference on my system -- clearer sound, wider sound stage, etc. After having the same experience mike'n'ike describes, I was very surprised at the difference it made. As with all things cable-related, the difference will be at least somewhat system dependent, so your mileage may vary.