Changes in DAC Presentation - Need Advice

As stated by many, a slow subtle change is what you would expect from burn-in or power.  You would also need to turn the unit on and off.  A momentary change, it sounds bright and then it sounds smooth or vice versa can happen once or can happen each time a unit hits temperature.  This usually happens with tube units but in a SS device that is on all the time, it has to be a sudden change in what is being fed to the unit or the unit is defective.  

Power generally doesn't have sudden changes.  One exception might be if your air conditioner is going on or off or some other unit that requires huge power.  This is often accompanied by the lights dimming for a second.  

I would be biased toward some other problem if a Mac Mini wasn't involved.  I have had so many customers have bizarre problems (DAC can't get IP Address, Cut outs, noise, skips in certain tracks, etc...) with Mac Mini's that I am prepared to believe they can and will cause any issue.  

An easy test would be to set up your server on another computer of any sort and see if the problem goes away.  

I have a voltage regulator on my line and notice a fluctuation of up to 10v.  It is rare that it will fluctuate more than 1v in a short period of time.  For a significant increase in voltage to occur, either an auxiliary power supply we need to go on/off line or a huge number of people would need to do something simultaneously.  Not sure this is terribly likely. 

Could be break-in, especially if it is going in the same direction (bright to warm or Warm to bright) every time your hear it change.  Do you notice a difference if the system is on for a while prior to listening or if your turn the system on fresh?  

Another obvious spot to look is the weak link in your audio chain which is that Mac Mini.  I talk to lots of people and more often than not, if something bizarre is happening in their system, a Mac Mini is involved.  Does anything else run on that Mac Mini?  If something else is taxing the processor, that could be your issue.   It would also explain why you might hear a change mid-song.  

Server matters in terms of sound quality even when not used as a renderer. You would be amazed home much noise the garbage power supplies and other bad parts can introduce. The core is also the more power intense function as it needs to tame a very volatile internet. The least expensive audio server will be a step up compared to a Mac Mini. Look at SGC or Roon for moderately priced options.

i can’t speak about the impact of a fiber output from a server to render.

I obviously don’t sell small green, I do sell Roon. I have never had a Nucleus returned unless someone upgraded to a better server within the 30 days.