Here's a site (with calculator)which shows the effect of adding multiple sound sources. You'll note that it's basically adding to the level of the first speaker 10 times the log of the total number of speakers playing simultaneously. For example, since the log of 2 is about 0.3, stereo speakers result in a 3dB increase; for all 5 speakers(log 5 about 0.7), a 7db increase.

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Enjoy the music, not the equipment.