Earlier in this thread, there was some discussion of Auro-3D. That motivated me to replace my receiver with one that had this capability. Initially, I was disappointed, finding it inferior to the basic Multi Channel Stereo setting on my Denon X4500H. Granted, I played around a lot with the volume levels of the various speaker pairs in Multi Channel mode, making sure that the sound was mostly coming from in front of me, less to the surrounds (but enough to give a wide soundstage), and even less to the surround backs and height speakers. But no amount of tinkering with the parameters of Auro-3D resulted in anything pleasant.
However, my initial foray into height speakers didn't work out so great for Atmos, either. I've since replaced them with speakers that are a much better timbre match to my base layer. So I decided to try Auro-3D again, and this time am MUCH happier! It's evident that the creators of Auro-3D have a similar goal in placing the musicians in front of the listener, though certainly utilizing the surrounds to produce a wide soundstage, similar to what I was doing with my Multi Channel volume settings. But their use of reverb to simulate a reasonably large performance venue is the difference I was looking for and could never have achieved on my own with simple level settings.
Interestingly, previous to Auro-3D, my setup seemed to somehow "suck out" much of the reverb added in the mixing/mastering process. I know I've heard this reverb (for the same recordings) on other systems and on headphones, but somehow couldn't hear it on my own. I'm not sure if it's the acoustics of my room, my aging hearing, or some combination thereof, but all of my music sounded very much like it was recorded in an anechoic studio, which was really bugging me, given how much money I've spent on it. (Yes, I know most music IS recorded in a more or less anechoic studio, but mixers are very careful to add sufficient reverb to make the final effect seem like a much more natural setting.) Auro-3D allows me to restore the level of reverb I remember hearing in my favorite recordings, giving me the missing piece that had been frustrating me.
Something I didn't anticipate: the reverb also seems to help me better "place" performers in the soundstage. I would have thought that reverb would have added to the problem of instruments seeming to come from a big jumbled mass of sound with no distinct locations. But instead, it's almost like the reverb is adding "location clues" for my brain. Anyone else experience this?
A big thanks to the awesome posters in this thread, who gave me a lot of clues on how to achieve improved performance without breaking the bank!