Doug- I am pretty sure that you are correct. It is additive, and not arithmetically. You would need an additional channel of amplification of course to power the second (third, etc)speaker. There are actually noise models that allow you to "add" noises of different dB level to give a total noise level. Same would be true for music.
Is sound "additive"?
If I kept adding speakers playing at say, 70 db to a particular room, would the sound keep increasing? Wierd question, but I was thinking about how it sounds when a room fills up and people are talking -- it just gets louder and louder (though some of this is can be attributed to people speaking louder to compensate for others' talking). I was going to try and relate this to the effect (in SPL) of adding multiple subs to a room. Would it just keep getting louder and louder?