Ok, measurement time. First is a comparison of the measured parameters of the new drivers in my subwoofer box.

Here are the measured parameters with the Infinity 122.7W drivers installed:

R=2.2 Ohms
Fs = 34.3 Hz
Qb = 0.99
Le = 1.97 mH

...and here are the parameters with the Alpine Type R 12D2 drivers installed:

Re = 1.90 Ohms
Fs = 37 Hz
Qb = 0.67
Le = 1.521 mH

The resonance frequency has increased, but the Qb has gone down significantly. Theoretically this should result in a tighter bass sound, but bear in mind we are talking about pretty low frequencies here.

The more significant differences showed up in the FR/distortion plots (see attached). For this test, I basically opened all of the doors and the trunk (to reduce the effect of the car audio transfer function), set the volume to my casual listening level, then used HolmImpulse and an ECM8000 mike suspended about a foot from the subwoofer to measure the FR and distortion. The FR/distortion curves for the 122.7Ws are displayed in blue, and the FR/distortion curves for the 12D2s are displayed in red. The graph shows that the output of the two drivers remains the same between 50 and 70 Hz. Below 50 Hz the 122.7Ws have a bit more output for the same input voltage, while above 70 Hz the 12D2s have a bit more output. Nothing that can't be addressed (if it actually needs to be) by a bit of EQ and perhaps a slight gain adjustment on the subwoofer amp. However, look at the distortion plots. Apart from a bump around 40 Hz (which is close to the resonance frequency of the 12D2s mounted in the enclosure), the measured THD for the 12D2s is 5 to 7dB lower than the 122.7Ws. And I expect this difference will only increase at higher volumes, as the 12D2s were nowhere near their rated Xmax during this test. Theoretically, this should amount to a cleaner-sounding sub, particularly at lower frequencies and higher volumes. Of course, whether or not that difference is actually audible depends however on how sensitive you are to distortion, and how much of that distortion is masked by road noise and everything else that contributes to what you actually hear when listening to your car audio system.

Subjective listening results next.