Some good ideas so far, but one thing that has not been mentioned is jitter.
Mizuno - You said in the OP that your dac is being fed by the S/PDIF output from Sonos. Sonos' S/PDIF output has a high level of jitter. Jitter results in frequency modulation that is audible as harshness in the high frequencies, which is exactly what you are reporting. If your dac does not reclock, then you are probably hearing the results of jitter. What dac are you using?
Bryon
Mizuno - You said in the OP that your dac is being fed by the S/PDIF output from Sonos. Sonos' S/PDIF output has a high level of jitter. Jitter results in frequency modulation that is audible as harshness in the high frequencies, which is exactly what you are reporting. If your dac does not reclock, then you are probably hearing the results of jitter. What dac are you using?
Bryon