Duke, I agree that using speaker equations can show that the lower cutoff frequency is a function of Qe. For example, if you assume a speaker design with a Qt of 0.707 with a driver that has a voice coil resistance of 5 ohms, a Qm of 3, 8 ohm impedance, and a Fs of 40 hz, then Qe at a DF of 1000 or greater will be 0.925 and give a lower cutoff of 40 hz. If you go down to a DF of 10, Qe is 15% higher and the lower cutoff drops 3 dB to 36 hz. But that's not a free lunch...
What you have to take into account is that the SPL ultimately depends on pressure generated by the driver. The factors affecting pressure are the driver magnet force factor, piston area, voice coil resistance and mechanical compliance. The SPL using these factors is inversely proportional to voice coil resistance. So decreasing the DF has the same effect as increasing the coil resistance so any -dB frequency drop by increasing Qt is offset by the reduction in SPL which is a wash. Otherwise, you could simply place a variable resistor in line with the driver which you can then adjust Qt to play with the frequency response in your room, independent of amplifier power.
What you have to take into account is that the SPL ultimately depends on pressure generated by the driver. The factors affecting pressure are the driver magnet force factor, piston area, voice coil resistance and mechanical compliance. The SPL using these factors is inversely proportional to voice coil resistance. So decreasing the DF has the same effect as increasing the coil resistance so any -dB frequency drop by increasing Qt is offset by the reduction in SPL which is a wash. Otherwise, you could simply place a variable resistor in line with the driver which you can then adjust Qt to play with the frequency response in your room, independent of amplifier power.