What is the proper loudness for listening?

optimal level totally depends on the situation... including other people in the room, your purpose, time of day, are you engaged in other activities, etc. ... and depends on the speakers, etc., too.

some speakers need to be "turned up" to come alive... I'd never want such speakers.  Others do very well at even quiet volumes; I'm constantly impressed by the dynamism and richness coming from the Klipsch Heresy at very low volumes, for example, and some of the most interesting listening experiences I've had with the Epi 100 over the years have been at low volumes late at night: they have clarity and sparkle.

@snilf Wrote:

Be aware, by the way, that "weighting" on SPL meters is crucial. Most are "A" weighted (note that the OSHA standard ditusa cites is given as "dBA").

See the tutorials below Decibel A, B and C:

Mike

https://www.engineeringtoolbox.com/decibel-d_59.html

https://www.vernier.com/til/3500

Thank you, @ditusa. These are very informative links.

Note that, according to your third link, "The A-weighted sound level discriminates against low frequencies.... In this setting, the meter primarily measures in the 500 to 10,000 Hz range. It is the weighting scale most commonly used for OSHA and DEQ regulatory measurements. The C-weighted sound level does not discriminate against low frequencies and measures uniformly over the frequency range of 30 to 10,000 Hz." This is just what I maintained in my earlier post, except that A-weighting actually cuts off at 500 Hz, rather than 100 Hz—which, of course, makes my point even more important.

However, I will admit to some puzzlement about the clause I elided: that A-weighting "discriminates against low frequencies, in a manner similar to the human ear." In my experience, audio system measurements are almost always given with C-weighting. Sometimes this is explicit, but even when the weighting filter is not specified, the numbers seem to be C-weighted. After all, we do certainly prize systems, and speakers, that produce sound below 500 Hz! To eliminate these sounds from the meter’s measurement because the human ear is less sensitive to them would not be what an audiophile would want to do. For one thing, low frequencies are felt as well as heard. Surely you (ditusa) are not advocating for measurements that ignore low—and high, for that matter—frequencies!

In any case, when I see the dB levels folks on this forum cite as preferred listening levels, and then try to match them on my system, I find that the higher levels (90 and above) MUST be C-weighted. My ears begin to bleed when I approach 90 dBA, but 90 dBC is just excitingly loud.

Some very good insight here, I enjoyed reading through this thread understanding there's always the factors of the individual person, system and room.

Personally, I find myself in 70-75db as a sweet spot for most types of music although I'll go slightly above that for classical to feel the scale and peaks. 

Those two items on each side of your head will be the guide as to how loud you can go with the sound. Your room will also dictate this and whether you're listening alone, what time of day/night you are listening, etc. If you plan on listening into old age as I am, then don't go overboard on the volume control!