I have had DACs with intermittent problems, some obvious, some more subtle. The first one was a Theta Cobalt, which would get crackles at times. The manual pointed this out and instructed to unplug it and re-plug it in with hand firmly on chassis, to clear static charge. Another is a Behringer DCX2496. It would also develop a crackling, sizzling sound. The cure was to disconnect and reconnect a ribbon cable inside the box. Fortunately that was a permanent fix. It also occasionally will implement a high Q cut at around 5k, a well known problem with no permanent solution. You have to turn it off and back on again, or re-set the digital stream to get it to correct itself. More recently, I heard a buzzing in my left tweeter. Very obvious when a flute played, or Link whistled for his horse in Breath of The Wild. After much trouble shooting I finally turned on the headphones and behold! The buzzing was still there. I had to re-set the digital stream multiple times, turning the source (an Ultramatch) and the DCX off and on again repeatedly to make it go away. It also caused the image to shift to the right. I don't hear the buzzing lately but I do perceive the image seeming to shift slightly to the right occasionally, making me suspect that something might be amiss, either with my ears or with the DACs in the DCX.
With all this I'm suggesting that there may be more subtle distortion issues that are intermittent with some DAC / digital source combinations. A good test on the bench doesn't mean it always works like that every single time you turn it on. I also have a cheap Denon receiver that mostly sounds very good to me, but on some days it seems to go flat and lifeless, and if I listen with my ear close to the tweeters and nothing playing, I hear some weird fizzling and whirring sounds when I perceive that lifelessness. Unfortunately I can't solve the problem by turning it on and off. I just have to wait for it to go away, which suggests it may be some kind of radio interference. I can also hear the fizzling through headphones when it's acting up, and it doesn't matter what the source is set to.
All just food for thought.