For years it's been known -- so long that, unfortunately, it's now lore with very little
in the way of written up explanation -- that in blind testing, two sets of speakers that are identical except for one being not even one dB louder than the other, the louder speaker will be judged to sound better.

One fact that generally supports this is that, within a range several dBs wide, the ears have a flatter response curve with louder sounds.