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.
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.