I would suggest ignoring the +3dB that comes from halving the impedance that the amplifier sees, but rather think of the amount of power going into each driver separately...and really it's best to think of the voltage being delivered and not the power (since a speaker impedance isn't flat). The amplifier is always putting out the same voltage, but the amount of current that flows is dependant on how the speakers are wired. In parallel, there are two equal paths so you get double the current (and thus double the power). When in series, you get half the current and each driver sees half the voltage, so you end up with the same SPL for a dual driver versus a single when the amplifier voltage is the same. However, if you can increase your voltage so that each driver sees its max power, then you still end up with +6dB maxSPL versus the single driver. You just gotta make sure your amp has the voltage to drive it.The interesting part of modeling two drivers vs one was that I could get the max spl of the one driver configuration on half the power. This left me wondering if this had something to do with box size increasing, or if it was just due to the increase in cone size.
So +3dB comes from doubling the amount of electrical power being delivered and then the other +3dB comes from halving the acoustic space for each driver. Two drivers in 1/4 space will play 3dB louder than a single driver in 1/8 space. Two drivers in 1/2 space will play the same as a single driver in 1/8 space.
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