As long as the drivers are located next to each other and reproducing wave lengths large relative to their dimensions, the efficiency increases with 10 log # drivers which is to say it goes up 3dB regardless in a pair regardless of how you wire them.
The configuration just affects the power delivered and therefore sensitivity. Wired in series each of a driver pair gets half the voltage so its output is -6dB down. But the two add in-phase so you get the same output for 0dB change in sensitivity. Since the load was doubled, current and therefore power is halved for the same output so efficiency is +3dB.
Wired in parallel you get the same +6dB in output, but power is +3dB from halving the load and efficiency has increased by the same 3dB.
Four drivers have +6dB efficiency regardless of how they're wired. In series-parallel they present the same 8 Ohm load and sensitivity is +6dB. Wired in parallel the present a 2 Ohm load, consume 4X the power, and sensitivity is +12dB - +6dB from quadrupling the power, and +6dB from efficiency.
Paired with an active cross-over that allows for different sensitivities across drivers and even the frequency band in one driver, a couple of bass drivers at +6dB sensitivity picking up another +6dB from floor mounting work really well.
The configuration just affects the power delivered and therefore sensitivity. Wired in series each of a driver pair gets half the voltage so its output is -6dB down. But the two add in-phase so you get the same output for 0dB change in sensitivity. Since the load was doubled, current and therefore power is halved for the same output so efficiency is +3dB.
Wired in parallel you get the same +6dB in output, but power is +3dB from halving the load and efficiency has increased by the same 3dB.
Four drivers have +6dB efficiency regardless of how they're wired. In series-parallel they present the same 8 Ohm load and sensitivity is +6dB. Wired in parallel the present a 2 Ohm load, consume 4X the power, and sensitivity is +12dB - +6dB from quadrupling the power, and +6dB from efficiency.
Paired with an active cross-over that allows for different sensitivities across drivers and even the frequency band in one driver, a couple of bass drivers at +6dB sensitivity picking up another +6dB from floor mounting work really well.