> I'd wager >99% of "surround sound" deployments would take more than a year to notice if they were transparently "downgraded" to stereo

I can immediately tell if anyone has messed with any knobs or buttons on my receiver or if any of the speakers seem off / wrong sounding. Maybe I'm that 1% but I can remember many multiple times people have been over for movies / TV and someone asks out loud "does the sound seem off?" and sure enough a kid or a clueless friend was messing with the knobs.

I'm not talking about EQ settings or dynamic range or anything. I'm talking about 'surround' specifically. I've also had a track record of noticing messed up audio settings quickly. But I probably wouldn't notice if my 5.1 started getting down mixed to stereo. At least for music.