Hi, 4yanx:
I haven't looked at the Vandy site to verify my comment, but as I recall, the upgrade from 3A to 3A Sig involves replacing the tweeter with the same one used in the Model 5, plus an upgrade to some of the components in the crossover. The mid-range driver in the 3A is the same one used in the Model 5, and this is also used in the 3A Sig.
I haven't looked at the Vandy site to verify my comment, but as I recall, the upgrade from 3A to 3A Sig involves replacing the tweeter with the same one used in the Model 5, plus an upgrade to some of the components in the crossover. The mid-range driver in the 3A is the same one used in the Model 5, and this is also used in the 3A Sig.