What is the proper loudness for listening?

I'd say as loud as you like it before the room and/or componants become overloaded.

Right now on certain records above 83/84 DB, the treble becomes harsh. I'm 98% sure this is a room issue and I'm working on that now. I was listening yesterday to a record that doesn't have that issue and was 75-88 for the most part but a few peaks up to 96-ish and crystal clear. 

Large room or small, the music sounds best to me when the measured peaks are coming in over 85 and under 90db. Loud enough for realism but not so loud that damage to hearing is a real concern. A sound meter app on your smartphone like decibel app is your friend both for determining good listening levels and frequency response aberrations from room acoustics, which in many rooms is quite significant, left unaddressed.

@laynes that could be odd order harmonic distortion that makes the treble hard to handle at higher volumes. It’s an artifact of many older SS amps. Tube amps and most modern Class D amps are less prone to that issue. @atmasphere talks about that all the time here if you search out his posts.

It is completely dependent on the system, room, and your tastes. I have noticed that the better my system has gotten the lower the volume sounds best to me. Typically I listen at ~ 75db. Most of the experience audiophiles with good systems have seemed to converge on this volume. My system is shown.