Actually this question is virtually impossible to answer with the information at hand. The sound coming out of your speakers can not be represented by a simple scalar value. It is not as simple as saying the output is 89db. The sound is however, a complex quantity where the sound possesses magnitude and phase. If the speakers, cables, electronics etc. were all exactly the same, it is theoretically possible they would all be in phase with each other and add perfectly. But because we live in an imperfect world where the signal paths would not be the same, some of the sound pressures could theoretically cancel each other out if they were 180 degrees out of phase which is exactly what noise cancellation scientists are working on right now.
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?