I believe there were some changes made to the crossovers (that's usually been the change with the v2s), but I was also under the impression that the M3s didn't have a crossover. Helpful, ain't I?
In any case, the v2 series is largely a designation that came about after a few years worth of changes and modifications that eventually lead to the decision to change the name. Depending on the speaker series, there may have been many changes (such as the EP350v3, which is a totally different design from the v2 and the original), or very few, such as the M50s.
