Heh, just like speakers, once you ask "what cable should I buy", you will get 18 different answers! lol. However, in all my R&D and testing, I have always found solid-core to just sound better than any stranded conductor cable. Audioquest pretty much uses only solid core (except for their cheaper power cord and speaker cables). Wireworld is not quite fully solid core - but it's not that bad because they align all the conductors in a row side-by-side instead of clumping them together in a single bundle.
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Since you're using Oppo 203, you could still try using pure analog out of it as well. You could try first with just left/right to get a sense of what it sounds like through the Denon AVR and then invest in another 3 RCA cables if you feel that's good enough for multi-channel SACD (if it's better than SACD through HDMI).
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Another thing that will significantly improve the Oppo quality is to get one of those linear power supplies from companies like Oppomod or Clones Audio. These are in the $300-400 range and will be a great improvement over the stock switching power supply. And it will allow you to do fuse upgrades in the Oppo.