Wrong again, brother.
1) I've never used S/PDIF on Kemper so my conclusion wasn't made based on that.
2) IIRC the Kemper's S/PDIF I/O is STEREO.
3) I had my O2R desk hooked up to a MOTU 2408 interface for years with ADAT optical (via expansion cards for the desk) and 2 RCA leads for the 2-bus I/O - 1 for STEREO in, the other for STEREO out.
There's a lot of misinformation on the 'net about this (surprise, surprise). A quick DuckDuckGo search yielded definitive quotes saying that RCA cables cannot be used / will not work for S/PDIF. They're not-ideal for the purpose but they do work. All these peeps are wrong. I'd even argue that for distances of under a metre, provided no clicks or pops appear, they are ideal if you already have legacy leads lying around and don't want to have to purchase new S/PDIF-purposed ones.
Yes. It is confusing. But you are using your Kemper wrong.
RCA cables on have a single center conductor with a grounding sheath around it. I suppose you can get a stereo signal by the center conductor carrying the Left and the grounding shield carries the Right channel but you loose any shielding. Maybe that is why you get degradation at longer lengths. I just can't finger out how a signal is being sent thru the shielding when it's suppose to be grounded-- like that signal would short out. This is a mystery.
We need an expert on the subject to explain.