Quite a while back while perusing the old, original blogs I saw some advice from Lukasz that I think has saved my sanity a few times. Thought I'd pass it on in case others may find it helpful.
Every once in a while my digital sound just seems to go downhill for no apparent reason - similar to what the original person described. The recommendation was to turn off the unit (DAC in this case), unplug it for a few minutes then put it back into operation.
I've done this a couple of times when sound has "gone south" and it's done the trick.
Rather like hitting the "reset button" I suppose.
Very interesting reading here, if you please:
http://www.lampizator.eu/Fikus/ARCHIVE_CONTENT_OF_LAMPIZATOR_BLOG.html