The slightly longer but very generalised answer is that the loudness at the listening position is dictated by the output level of the amplifier, the efficiency of the speaker and the size of the room. The dynamic range of the source material is also a factor in this as heavily compressed recordings will sound louder.
I'd recommend understanding these relationships if you're trying to spec a system to meet your needs i.e. realising you don't need a 300W amp if you have an average sized room and reasonably efficient speakers. But if you want to protect your hearing you won't get close enough to the correct answer without measuring.