Oppo themselves have stated that if one is not adjusting the picture controls in the player the bit depth output makes no difference in the output. But what does happen, is some displays handle different bit depth inputs differently. Some doing expanding or compressing the range depending on what their native processing format is. That's probably what you're seeing. When you give your display a 24-bit signal, it is scaling it up to 30-bit with some artifacts. But when given a 30-bit signal it doesn't perform the expansion. Oppo does say to play around with the setting to see which gives the smoothest gradient on a particular display.
Yeah, EMI/RFI can corrupt the signal in an HDMI cable, which is what causes the sparkles, or pops/clicks in the audio. If that's happening, then get a better cable. If it isn't happening, then the cable you have is fine. Digital artifacts are pretty obvious; when things go wrong they do really wrong. It isn't like with analog where things may be a little fuzzy, or some frequency response is lost, and you can't be sure if you're really getting the best picture/sound.
