I’m into this, especially sound quality. I have a top-of-the-bottom/bottom-of-the-middle 5.2.4 setup for movies and listen to music on that same space as well. I have made solid progress on acoustically treating my space, and it sounds pretty good.
My wife, who has done all the A-B testing with me and understands what high quality equipment and a well-treated space brings to the table would be perfectly happy watching Netflix on her phone speaker.
I know for sure she does not hear things like I hear them. I have a couple of demo tracks I use to evaluate changes in my system, and I have described to her what I’m listening for (soundstage depth, for example), and she cannot distinguish whether or not this quality exists. So, I think one part of this is that there is something cognitive going on that she and I perceive things so differently. Another thing that I think is different between us is the way music affects us. For me, music is an emotional experience and ties deeply with my memories of events/time periods/feelings. When I hear a song, I know the artist, facts about the band and its members, the name of the song, the album, and I can describe the album art. My wife, on the other hand, usually can’t remember the names of her favorite songs or who the artist is. And, like, no shade on my wife for this at all; I’m just saying we experience these things much differently, and I think that may be illustrative of the differences your seeing with other people too.
Additionally, as far as diminishing returns go, I think a lot of people do not understand the importance of acoustic treatment. You’re listening to your room, not your speakers. You can’t out-speaker a bad sounding room at any cost. If you think you want new speakers to upgrade your sound quality and have given zero thought to room acoustics, you should do a little investigation before upgrading your equipment. I think that money is better spent elsewhere first.
Once you have a good sounding space to listen, I really don’t think most people need to spend any more than 10k (at the extreme upper limit) for a pair of speakers for 2 channel listening. That said, I have been in a fantastic room with a $15k pair of Sonus Faber speakers, and I have been chasing that feeling for a couple of years now.