It needs to be as loud as you need it to be, no more and no less. I'm also in the camp that says each type of music and recording can determine what that level should be.
Every recording was mixed and mastered at a certain volume. Good engineers will then check and make sure that the end product holds up at different volumes. That is not always the case, though. To get it to sound right often you need to match the levels used during the mixing and mastering process.
However, in spite of everything, in the end the sweet spot is were the music hits you as right. For me it is usually 80 through 110. Rock and pop is generally louder, jazz in between and folk or singer song writer lower.
Every recording was mixed and mastered at a certain volume. Good engineers will then check and make sure that the end product holds up at different volumes. That is not always the case, though. To get it to sound right often you need to match the levels used during the mixing and mastering process.
However, in spite of everything, in the end the sweet spot is were the music hits you as right. For me it is usually 80 through 110. Rock and pop is generally louder, jazz in between and folk or singer song writer lower.