SPDIF is a system; the transmit circuitry, the cable/connectors and the receive circuitry. If you are using RCA connectors, you have deviated from 75 ohms right there. Most transmit/receive circuits are also not terminated correctly (regardless of equipment price). The cable length also matters. Jocko, etc. all think that 1.5m or greater is the minimum length. Dan Lavry thinks it should be as short as possible. I would tend towards the >1.5m. Of the cables you tried, the Stereovox has a good reputation for being technically correct. You can search diyaudio and diyhifi.org for more technical details. Dan Lavry posted on hf in a cable thread regarding the length issue, but Jocko is/was an RF engineer and knows what he is doing (if you can get him to tell you ).