Your TV will automatically upsample any 1080p content, so there's no benefit to relying on the TV for video processing. Avoid using Optical or eARC for audio—both are bandwidth-limited and won't deliver full-resolution formats like Dolby TrueHD or DTS-HD Master Audio from Blu-ray discs. While eARC has higher bandwidth than ARC (up to ~37 Mbps vs. ~1 Mbps), it's still insufficient for full bitstreaming of advanced codecs, and the SSP-800 does not support ARC or eARC input natively.
Instead, use an HDMI audio extractor to separate the audio from a 4K signal and send it to your SSP-800 via HDMI, I recommend the HDFury Vertex²—it's a highly flexible HDMI matrix and splitter that can strip audio cleanly and pass full 4K HDR video to your display while routing high-resolution audio to your processor. I used one with my SSP-800, an HDFury Vertex 2nd, and it worked flawlessly.