What DB level is best for judging audio equipment?

+1 @elizabeth  The listen in the range of 65dB to 77dB according to my preamp volume control, which appears very accurate. My smart phone meter generally shows the transients to go as high as 82dB. I intentionally do not listen louder than 80db on the preamp (ever). I simply want my hearing to last as long as possible.

I also play music at lower levels.
Those stupid headphones of the 70's were my undoing...
Bob

Listening at lower levels was one of my auditioning tests, since I live in an apartment.  In fact, it was a major distinguishing element between the various loudspeakers I listened to in my most recent round.  Some needed to be played at high levels to really shine, others did not. 

I didn't audition enough speakers to tell, but it seemed like the ones with the more exotic materials were the ones that needed to be pushed.

It might also depend on room acoustic.  I have a bit of slap echo in my living room, that is difficult to get rid of.  Playing too loud makes sound worse, less clean - most likely from increased number of still audible reflections.

Smartphone meters are rubbish. Most cannot register SPLs above 90db and they tend to display a level that is 5 -10db lower than actual c-weighted SPL. The problem is you cannot judge the accuracy of your smartphone mic and app without referencing a professional, calibrated SPL meter.

As for what level is best, it all depends on your listening habits. Better to ask what sort of gear works better at lower or higher levels. All else being equal, higher sensitivity speakers will sound more dynamic at lower levels than will low- sensitivity speakers.