What is the proper loudness for listening?

Though this won't include Rock or some Acid Jazz, it is simple for me. The perfect level is found when you look for the soft passages, often found quite noticeable in classical music, and set that at a comfortable but yet soft volume i those portions of the piece, the rest takes care of itself. This after a while and a little practice, this  will come naturally. For those that think that music is good if it is loud, I can only offer my pity, Volume has noting to do with sound quality, with one exception. My speakers and I assume most quality speakers don't even 'Turn On' till they get a certain power level. This means that Yes they will play sounds at lower power levels but their excellence doesn't shine until that minimum threshold is reached. This is going to be different for every speaker made and is like in my example above, you set your volume higher to achieve this quality threshold, you may be pushing the limits of crazy when the more powerful portions of a piece come along.
What does all of that mean? It's totally up to your own particular taste and what wifie will permit before she gets the cast iron skillet out to clobber you with.

For me 70 - 85 dba and I am more than happy.

I am surprised how loud some are able to listen.

I use a dB meter, C-weighted, fast response.

75 dB peaks when my wife is sleeping in over the weekends.

85 dB peaks after she has joined the living, and it is LOUD to me.

I have learned that the best way for any system is to turn it up till it starts to get harsh, then turn the volume down till everything comes into FOCUS like a lens.You can tell pretty quickly. I heard a LOT of systems at AXPONA 2023 with my wife and we both thought most systems we heard were too loud for the room. But, there are people coming in and out, talking, etc. So it is limited to say the least.

                             How LARGE do you like your music?

     Unless a system can cleanly reproduce all the information (well engineered/recorded in an environment with good acoustics and mic placement) in a quality cut, to the original/intended dB level; It's listener will never hear that information.

              ie, regarding, "depth": the reflection off the venue's back wall.

      Of course: familiarity with the venue in which a recording was made,  would go a long way with regards to recognizing whether what one's hearing is actually accurate.    NOT that that's a necessity, when it comes to the enjoyment of one's music, BUT- having that knowledge, one can be confident that their other recordings are also being faithfully reproduced.

       'Checkerboard Lounge Live Chicago 1981' (on vinyl) is a favorite of mine, far as being able to hear the room, especially between songs.

        Especially, in the softer cuts of Diana Krall's 'Live in Paris' vinyl (45 RPM/180 Gram), I find the Olympia Theatre's back wall reflections nicely reproduced (with accurate depth).

         Back the the size-of-your-music thing: I turn my sound up slowly (a song at a time, to acclimate the ears to higher dB levels, without having them shut down), until my image height reflects where I imagine/know the performers to have been, when recorded.     Again: a system has to be able to reach that level cleanly/without distortion, or: it's just LOUD (iow: noise).

        It's been my experience: seated in the better/more expensive, front and center seats; it's easy to hear and locate individual voices (human or instrumental), on a stage and seldom would the level be low enough for some to consider, "safe enough".     Yet: no one complains, because it's clean sound (just big).

                    OF COURSE: different strokes, for different folks.

                                          Happy listening!