Thanks for your help! Still working my way through the differential diagnosis...
When I run a test, I have -- all -- components in the signal path (even the ED1A ADC) at the same sample rate. I also verify that the D10 and E50 are displaying the rate under test.
Of course, I first tested with a loopback: PC -> (USB) -> Topping D10 -> (SPDIF) -> Topping E50 -> (Analog) -> E1DA -> (USB) -> PC to make sure that worked.
I inserted the MCHStreamer and Ubuntu into the path and it worked.
I then replaced the MCHStreamer with the U-DIO8 and it failed at 192 KHz.
I then generated a signal on the Ubuntu system and played it back through the system using these commands:
$ sox -V -r 192000 -n -b 32 -c 8 square1k.wav synth 30 square 1000
$ aplay -D hw:1,0 square1k.wav
and that test was successfully. This means the second half of the processing chain works -- I can get a signal from Ubuntu through U-DIO8 at 192 KHz. So, the U-DIO8 output and E50 input SPDIF is working.
So, this suggests that the problem is the SPDIF input -- either the cable, or the unit, or some incompatibility between the D10 and U-DIO8.
Next, I did something similar to what you suggest -- I sent a signal from the PC and recorded it on Ubuntu, and then was going to copy that file onto the PC to analysis. This will check the first half of the path, but I haven't copied the file off and analyzed it. I paused testing and was waiting for their response to my support request. Maybe they have seen this before and know the problem.
(I had also used an oscilloscope to look at the SPDIF waveforms between the D10 and UDIO8 and I'll say that the cable provided with the U-DIO8 isn't helping (the D10 signal at the end of a 1m generic cable looks better than that same D10 signal at the end of the U-DIO8 cable (have to measure at the D25 connector), but I think it is good enough.)
The D10 is simply an SPDIF output for the PC for test purposes. In the actual system I will be building, the SPDIF source will be a blue sound Node and there will be 3 Topping E50s for six channels on to the amplifiers... the signal path will be:
Node -> (SPDIF) -> U-DIO8 -> (USB) -> Ubuntu (CamillaDSP) -> (USB) -> U-DIO8 -> (SPDIF) -> Topping E50 (x3) -> Amplifiers -> speaker drivers
This is replacing a decade old system that used a heavily modified DCX2496 for the crossover, equalization and time delay. Now I will have a much better DAC (with the E50s) and will have much more DSP power with CamillaDSP. Well, assuming I can get it to work!