Can hardware change speaker phasing over time??

Please God no! Not a receiver! Lord have mercy!

I liked this thread a lot more when it was about Will Farrell movies. Volcano Man!  https://www.youtube.com/watch?v=AdW6BBF22AY

There’s parts that are really, really well done. There’s parts that are heartfelt and inspirational. There’s parts that are basically fart jokes. Because it is after all a Will Farrell movie, so get real. The songs are the highlight. The opener is as good as anything from ABBA, and I actually like ABBA, and this is that quality, only funny. A lot of the songs have double meanings. Jaja Ding Dong seems like a funny Icelandic ditty until you catch "my love for you is growing wide and long" which if you think about it is pretty explicit, and the running joke about him being in love with his sister, Pierce Brosnan being a Lothario who fathered half the town. Also the contest political stuff is really well done, the banker pulling the strings, etc. So basically about half good gags, half good funny music, half potty humor. Typical Will Farrell, up there with Blades of Glory. (What do we have that the others don't? Matching junk? Right!) 

There’s simply no way any hardware caused a polarity switch in the speakers. Other than elves, I mean. But you do have a receiver, inside which all bets are off.

When properly set up the difference between in and out of phase will be more than just vocals being focused. When playing a mono track in phase everything should be coming right from the center. Nothing anywhere else. When playing out of phase the mono track should be coming from everywhere. Not only is nothing coming from the center, nothing is coming from anywhere. The sound is so diffuse there is nothing you can point to anywhere as the source of any of it, vocals, instruments, none of it. I forget the Stereophile CD but the XLO CD has a track recorded mono to do this test.

From what you’re saying its almost certain one channel did get reversed somewhere, some time. To track this down, follow the signal path. Plug earphones direct into the Mac mini. Then direct into the DAC. Use an adaptor, or sacrifice a cheap interconnect to get at the wires and hold them on your headphone plug. These connections only need to be good enough to hear if its in or out of phase. Which should be pretty obvious.

My bet is one of your three digital devices (I’m assuming the receiver includes a digital processor of some kind) messed up and needs a reboot. But mostly because otherwise we are back to the elves.

There really is only one explanation for mysteriously reversing wires like this. If you want to avoid further problems I suggest you pay close attention to this video, lest the situation escalate. https://www.youtube.com/watch?v=wW370Aj8E6g