Can hardware change speaker phasing over time??

@ stroud27612
perhaps let's work through a troubleshooting procedure?
Your Denon receiver, is that your volume attenuation to your A21 amplifier?

Have you tried using another source? Like a CD player with it's own DAC for example?
Try switching to another source after you return the cables to your speakers in correct phase?
Did you reboot the computer?
Sorry, you may have done all of this already - I'm trying to help you get to the root cause, by a process of elimination.

@rixthetrick Very good ideas!   I tried this tonight.  I love that right now my living room floor is covered in wires and loose CDs, just like the old days, all in search of Truth!  (If only there were full ashtrays, empty wine bottles, and some young woman's unmentionables scattered in there, too.  Everything that made me love 2 channel!)

So I got an old dusty Playstation One out of the basement (Stereophile Magazine's #1 all time CD player) and plugged the right and left channels directly into my Parasound Amp.  So now we have bypassed the DAC and the Denon completely.  After adjusting the levels on the back of the Parasound, I played multiple Stereophile test disks phase check tracks (both Fender bass guitar and even dogs!), and they all sounded better and proper with the "out of phase" wiring instead of "in phase."  At this point, you are all rolling your eyes and saying one of his wires is crossed, but I have checked that many times.  I also played other CDs, including Rebecca Pidgeon, that normally produce great imaging, and they are all better after I rewired that one speaker.   It's interesting that the Denon, when included in the stream, still believes I have that one speaker out of phase, which I do if you just look at the speaker wires.

Anyway, I'm just going to put it all back to normal with the one speaker "out of phase", and keep grooving on some Nick Lowe and Cowboy Junkies.  So strange because the regular wiring worked for me for years, but suddenly we have this alternate sound that cannot be explained, but it can be fixed!

Tonight I'm going to drag up a different pair of speakers and see what happens.

try another amplifier?

The mystery deepens! At lunch I hauled up my father’s old Mirage speakers and hooked them up to the full system, wired "in phase", and they sounded and tested as "in phase." So it has to be the B&W speakers.  I put the B&Ws back in the system, and they still only sound "in phase" if wired "out of phase."

Tonight I will hook them up to a different amp and see what happens.

And I have now created a special curse for B&W because you cannot use banana clips on them.