So, perhaps it IS the auto-mute function. What a PITA. Is there any purpose for the auto-mute? So, maybe the dac is working just fine, which is good. It's just a huge bother when conducting REW measurements. Thankfully, turning the dac off and back on for each REW measurement seems to create an accurate and trustworthy frequency response measurement for the R channel. Also, if I use Speaker Cal for the pink noise rather than Full Range, I can push continuous sound out of the R monitor and thus be able to synchronize the loudness of each monitor (adjusting gain on back of monitors). If I switch to Full Range pink noise, I get at most a second or two of sound from the R monitor (L channel and monitor are NOT affected), and must turn off and back on the dac to get any additional sound from the R channel.
So, if the dac is functioning (it sounds good, especially after this recent REW measurement and EQ filtering), I suppose it's worth keeping. But I still wonder . . .