The main differences between each model can probably be summed up in a couple paragraphs:
The small two-ways (one π / two π) are high-efficiency cone/dome speakers. They're great as surrounds or secondary systems. The two π tower is capable of deeper bass.
The three π and four π are matched-directivity two-way speakers. The six π and seven π are constant directivity cornerhorns. More information on the significance of each design type is in the whitepapers listed below. the eight π is sort of a hybrid, and can be thought of as an entry level constant directivity cornerhorn, although corner placement isn't absolutely mandatory on that design.
The biggest difference between the three π and four π speakers is the the midwoofer families available. This is also true for the six π and seven π cornerhorns. The three π and six π models use the Eminence Delta 12LF woofer in the entry level model, or the Acoustic Elegance TD12S woofer for the high-end upgraded model. The four π and the seven π speakers use the Eminence Omega 15 woofer or the JBL 2226, as an upgrade option. All models can use an Eminence PSD2002 compression driver or the optional B&C DE250 driver.