Does Aurender do Processing on S/PDIF Outputs?

“Does anyone know if an Aurender N100C does signal processing on its S/PDIF outputs”

In one word YES!  The SPDIF audio output differs from asynchronous (USB) output. With SPDIF output, the clocking / processing is done within the server as opposed to USB output where your DAC clocks and does rest of the processing of incoming signal. 

The good thing is N100C comes with both USB and SPDIF outputs. If you don’t like what you are hearing from SPDIF output then try USB. Which output sounds the best is largely depends on how good is your DAC with its implementation of USB and SPDIF protocols. One must try both in the context of your system to know. Basically it comes down to the implementation and which component has superior clocks. 

Yes, I tried the USB output too.  Surprisingly, I cannot discern any difference at all between the USB and S/PDIF feeds from the streamer!  Only between the two streamers on S/PDIF.  I mostly did not expect the difference because the Benchmark DAC tries to go to great lengths to eliminate the effects of source jitter (at least).

I am also trying to discover a technical reason why something with the S/PDIF signal would cause an apparent frequency response change, rather than something more subtle like clarity.

Have you paid attention to the digital cables you are using?

In this instance they are all identical.