this text below is what i found some month ago on the Forum which makes it clear
what you can do with spdif, think it was posted by Kemper support
" the head/rack/stage all automatically lock to the supplied sample-frequency applied to it's IN. So if you only want to use S/PDIF in 1 direction only, the head/rack will 'slave' correctly, but this is not what is commonly known as slave mode.
unfortunately the head/rack cannot slave it's OUT-clock to it's IN-clock due to a hardware issue in the used chip : so it's not possible to do a S/PDIF 'loop' : PC -> KPA -> PC (which would be needed for digital re-amping, and which is commonly called 'slave mode') "