I figured that my test can only be complete when I compare my findings above with how it could be, how it has been for the longest time. So I made a rollback to the fastest Kemper OS in the history of the world, version 5.3.1 ?
Constant Latency is indeed constant at 4.1 ms for SPDIF and 5.6 ms for the Main Output.
That's interesting. I did my tests with the PowerHead. Maybe the Stage has different SPDIF hardware (as somebody suggested here), since it has the slave option, which I don't have.
Lightbox, is your Constant Latency really constant though, is it 7.528ms every time?
I just did some testing with the newest Kemper OS 7.3.2 at 44.1 kHz
Test 3: Constant Latency ON - Take 2
Results:
What shall we do?
I'm glad I'm not the only one anymore, who cares about this 
I did a few SPDIF latency tests in August 2018 (with the newest OS at the time, I think it was 5.5.2) , which looked very much like the one in post #27 from Refuge Studio. I got latencies between 7.8 ms and around 10 ms, all with clean profiles. Constant latency was around 10.6 ms, but strangely enough, I never got it to be constant. It changed all the time.
So I called up the Kemper support hotline. They didn't know about this issue ("10 ms? That's worse than the old Line 6 POD!"), but the man on the phone said it must be part of the new update, where they added the 96 khz sample rate, which doesn't give you a better sound, since it's just upscaled, but people requested it. And he said something like it's technically not possible to have 44.1 khz AND 96 khz sample rates AND low latency. He was kind enough to send me the last OS (5.3.1) where SPDIF has low latency (around 3 ms; constant latency around 4.9 ms) , and he said he will put it on their Bug Fixes list.
In August 2019 I did new tests with Kemper OS 7.0: A clean profile was 7.5 ms late, with "Constant Latency" on it was 9.2 ms later than the original signal. So apparently they didn't fix it. I called up the support hotline again, asking if I was the only one who uses SPDIF for recording, since it's unusable in this form. From the way it sounded, I apparently was the only one. He said that most studios he knows use the Main Output and a cable (that has it's own sound, and can be faulty, and needs space) that plugs into their interface preamps (which have their own sound), where they have an additional AD/DA conversion (which has it's own sound, and adds latency). But he said he will put it on the Bug Fixes list.
So here I am today, not knowing what I should do. I can not upgrade Rig Manager, since version 2.1.61 is the last version that works with my old low latency 5.3.1 KemperOS. But I am not sure the old Rig Manager will run on a new Mac with OS Catalina, which I will need to upgrade to in the near future.
I really love my old fashioned low latency SPDIF recording, I can quickly choose between stereo recording, or DI on one track and Master on the other, which is something I do all the time. But in the current form, I think SPDIF on the Kemper is dead. Is there anybody who cares for groove that uses it?
They are all cables that came with external hard drives, cameras and printers that I've bought over the years. All 7 cables work fine with these other USB devices.
But I am willing to get a new "over-par new-stock cable" if that's what it takes. Any recommendations?
I am having trouble connecting the Kemper to my Mac Mini. I've found 7 USB cables in my household and only 2 of them establish a connection to the rig manager. And even then I am often running into problems (lost connection, Mac still thinks the profiler is connected after unplugging the cable, sometimes the one but not the other cable works, having to restart the Mac to start all over and pray for the connection to work this time...)
Can somebody who connected the Kemper with the USB slot of his computer only once in his life and never ran into any trouble afterwards tell me which cable he bought?
