Rbj. Your question is a very observant one. If you have followed my threads on the single driver subject, you will know that I have been using a network like the one you describe for my own speakers. Yes, the network does lower the impedance spike at the resonant freq. and allows the amp to control the woofer more comfortably at lower freq.s, thereby extending useable bass response and reducing phase shift. However, trying to use long Xmax with a tight cone suspension merely raises the Fs of the driver and takes you away from your goal of lower bass response. The method I used to get good results was to use a network, and tune a transmission line to a freq. half that of the driver Fs. This allows the T-line to handle the output of the lowest freq.s at a higher SPL than the cone would produce, and with shorter excursions. In my particular system, I achieved a full octave deeper bass before rolloff, with this method. In my network without the capacitor, the high freq.s in a single driver system are also improved similarly, with an extension to the high end gained by reducing the gradual impedance rise that is commensurate with rising frequencies. T-line tuning is a bitch, and has some problems like comb filter effects, but they can be minimized with some work. I think that if more people employed a design like this, you would see more single driver systems being accepted as true full range systems. The nominal impedance value of the driver is slightly lowered so an amp with some flexibility in this area is required. Good question.
