[jedcred]

I have a Constantine+ DAC connected over optical to a computer, with a WooAudio WA6 amp that I got as a pair from a friend.  It's been working fine for about a year now, but recently it has developed an issue where, at some random interval of being on (though not necessarily in use, but somewhere between 15 minutes and an hour), the left channel will begin to decrease in volume.  Usually I notice this after starting the music back up, though lately it's been happening while I've been listening.  The way I've found to remedy this quickly is to flip the power off on the DAC and flip it back on.  It'll be fine for some time until it happens again.
 
I opened up the Constantine to see if I could spot a bulging capacitor (at least one obvious issue that would be visible) but there wasn't one that I could see.  I'm somewhat handy with electronics (though it has been some time since the last circuits class), so swapping out parts would not be difficult.
 
Has anyone experienced this kind of issue before with this particular DAC?  Does anyone have any suggestions as to some diagnostics I might attempt?

 

[brunk]

This issue sounds like it belongs to the tubes, either in the DAC or the amp. Have you verified your amp is ok with a different source? Have you tried reseating and/or switching different tubes in the DAC?

 

[jedcred]

So I did, at first, swap the two 6DE7 tubes on the amp from side to side, under the assumption that each powers one channel (something I was unable to verify from the manual, but I assume is the case) with no effect.  Also, power cycling the amp doesn't seem to have any effect.  Unfortunately, I don't have any other DACs handy to test the amp in isolation. 
 
By the way, this particular DAC doesn't have any tubes.
 
One thing I will do tomorrow is test it over USB (I was using optical for the better quality) and see if the problem may exist from the input and not somewhere farther up the chain in the DAC itself.