How loud is loud?

Seems like a rhetorical question, but I'm curious what other folks think is "ok, now that's loud"

For me, if something's hitting 100dB as measured from where I'm seated while listening, and not just for a brief moments, but with some regularity, that's loud.

I used to listen at higher average volume than I do these days. Typically, I'm finding that at the right volume, the recording is more nuanced then when it's running full throttle.

If I'm not mistaken, the late Peter Walker observed that every recording has it's one correct or optimum playback level. I think generally he was correct, though once we are willing to forge optimum there's a range of acceptable.
My average is between 85db - 95db at my listening chair for hours with out any fatigue.
Check with the local police department since they are governed by each town ordinance which spells out in db's what is loud and when it is loud. Then get a sound meter and have someone go outside when you are listening at levels you think are okay. Then, call the police and report yourself since I am sure you want to be a good citizen first. Yes, I have had the police come calling. No, they did not come with a sound meter, so they were confused when I mentioned they had no way of really knowing what loud is. Yes, they felt I was a wise guy, but did not arrest me.
**The cleaner your system is, the less loud it will seem.**

This is because the human ear/brain system uses higher ordered harmonics as cues to determine sound pressure. In this regard the human ear is actually more sensitive to such harmonics than fine test equipment: we can hear harmonic distortion that we have troubles measuring.

If your system sounds loud at 90-95 db at the listening chair then it is an indication that there is excess harmonic content that is not part of the original signal (unless the signal itself has added distortion too). This can come from the electronics, resonances in the playback system or hard surfaces in the room itself.

If you are running an SET and it seems loud at less than these levels its an indication that your speaker is not efficient enough to take advantage of the SET's ability to have low distortion. Instead, it is being driven hard enough that the higher ordered harmonics are showing up on peaks/transients- this causes the amp to sound like its playing louder than it really is and contributes to the sense that it is playing loud.

It is these harmonics (particularly the odd orders, 5th, 7th and 9th) that are why transistor amps tend to sound brighter and harsher than tubes as well, despite these harmonics often being at seemingly vanishingly low levels. But our ears use these harmonics to calculate sound pressure, so even the slightest change in this area can be heard.

An additional complicating issue is that our ears are tuned to birdsong frequencies (Fletcher-Munson). Musicians like to make sounds where the harmonics fall into this very sensitive range as well. Its no easy task that sound systems have to play everything right without contributing in a negative way at these frequencies!
I noticed that in spite of Fletcher-Munson curve some gear still plays lows and highs at low volume while other gear quits and sounds midrangy. Also, room itself plays role. In my room playing soft sounds cleaner with better imaging. I suspect it is reverberation in the room that is causing multiple echos - still audible at higher volume.