I bought a COAX/SPDIF converter, and my Kemper sounds a million times better using SPDIF into my Apollo Twin instead of TRS/TS. Until proven otherwise, I don't think I'm experiencing a problem. It's just that SPDIF sounds better than analog/digital conversion.
You own an RME Ff800.
If you go Kemper spdif out to RME spdif in there should be no difference to the analog way, Kemper analog to FF800
I can’t here a difference with Neumann KH150 and Sennheiser HD800s or AKG 812