Here are the IMD test results at 6 pints from -50 dBFS to 0 dBFS in 10 dB increments. The dashed line is the much higher resolution output of Topping DX7s created over USB from my computer. The red is also Topping but now driven by those few steps using Volumio. It kind of, sort of shows the "hump."
Katana and ApplePi show better results across the board and without that hump.
View attachment 14947
The data is pretty coarse though. I would need to add more steps to be definitive.
As it is, it points to something specific to ESS DAC and how Topping buffers its output.
Boy, manual testing is so time consuming!