I have said on many occasions that for every room, system, and track of music there is a perfect volume level.
It's an observation that's probably worth repeating. Why? Because it's personal, contentious, and seemingly without a standard reference.
I would argue that for most recordings, we actually have a standard.
A vocalist is the easiest example. Too soft and the singer is too small. Too loud and their voice becomes the size of a barn.
When that singer's voice is the correct size you know you've hit the proper volume level. This, despite any differences in our hearing, tastes, or opinions.
What's interesting to me is that unless you're always a single listener there is no fixed level per track. If I always play a certain piece of music at 42 on the BHK preamp, that works well until there's more people in the room than just me. More bodies equals more damping and the need to crank 'er up just a bit.
When you are critically listening to your system, it's helpful to know there's a perfect level of volume to hit for every single track of music.