Placing woofers in series will double the resistance and inductance. This will cause a more steep frequency response roll off on the upper end and will show how sensitive the drivers are at half their standard drive level sensitivity rating. In theory, half the voltage applied to a driver will result in half the sound pressure in the output (-3db). But if two drivers outputs are combined, in theory, output should be doubled - thus cancelling out the effect that halving the applied voltage generates. That's the theory. In practice, the summation of separate but identical woofer responses is somewhat complicated because the way sound pressure waves combine in free space depends largely on the size of each sound source relative to the frequency wavelengths of interest, the relative proximity of the sources in space to one another, and the location in space that response is being detected. In general, you will find that the parallel/series arrangement of multiple drivers can improve overall acoustic efficiency and "apparent sensitivity" at lower frequencies. The essential reason for this is that more driver surface area improves acoustic coupling of the energy source (voltage) to the media being acted upon (air molecules). Try connecting four identical woofers with two series pairs in parallel. You should witness increased output at low frequencies over a single driver for the same applied voltage at most voltage levels Again, however, what is detected in response is heavily dependent on the spatial and frequency characteristics outlined above. Electrically, you should have the same nominal net impedance as one driver but acoustical coupling improvements should provide increased output for the same applied voltage and current draw (the series/parallel arrangement of the 4 woofers should restore the net final inductance value to approximately that of one woofer). All of the above ignores issues pertaining to the driver's inherent non linearity caused by electrical/mechanical energy loss/storage. For the purposes of the above explanation, one should just assume a "linear" driver.
