The big difference in the IIIa and 3.3 was a change in the ribbon. The ribbon on the IIIa is a tad shorter (the housing is 60" while the 3.3 is 60.5") and wider thus having a lower impedance. This required a 1 ohm resistor to bring the impedance up to 3.3 ohms. The ribbon in 3.3 is the same that is used in 3.7 and 20.7 and all the models in between. I believe the mid-bass panel is the same.