We are working in different configurations with IZ Radar Systems, RME Digiface, Apogee's and digital consoles. When set as master the Kemper's clock signal via spdif never degraded or influenced the sound quality from any of the devices or of the whole system.
The first generation 2012 Apollo Interface (mid class converter btw) seems to have issues with external clocking. This is not the fault of the clock source (Kemper). The issues are rather caused by the design decisions for the clock section in the Apollo, From a gearslutz Apollo User review:
With all the conversion tests I had the opportunity to compare the Apollo on its own clock and on external clocks. I found the Apollo to be quite sensitive to clocking and I found it to sound noticeably worse when clocked to some converters. Now, I am from the school that believes a better wordclock cannot improve the performance of the internal clock in the unit, but that external clock can, however, change the sound of the converters that are clocking from it.
This definitely seemed to be true with the Apollo. While clocking it from the Digidesign clock noticeably degraded the sound, I found the smoothness, depth, tightness, and punch of the Apollo's converters to improve when clocked to a Big Ben, at the expense of a little stereo width.