But what's receiving the S/PDIF? How do you know it's not the thing that is looping a buffer?
There is a S/PDIF receiver chip decoding the signal to I2S followed by an ASRC and the DAC chip: till there is a valid S/PDIF signal at the
input, the thing keeps moving data refreshing constantly the analog output. So, no looping buffers there: first in first out.
The only way to understand something is by measuring what data the S/PDIF carries out the XONAR after a stop while playing music.
No problem whatsoever if MC is stopped while the track is silent.
The XONAR uses muting relays on each analog output so the problem can't be seen there.