Mike,

At the risk of making a sweeping generalization, the issue is less about when and more about why. In other words, I think that the distinction your sources have noted between European Rennaissance and Enlightenment warfare may have have more to do with the underlying source of conflict. I suspect that a war which is in large part based on a conflict of ideology --Catholicism versus Protestantism in the 30 Years War, for example--will tend to be much more nasty than one which is largely focussed on using force to legitimate a claim to territory or establish one's hegemony--War of Spanish Sucession, Seven Years War, and the War of Austrian Sucession, e.g.
The first three Anglo-Dutch Wars (wars to establish commercial hegemony) belong to the earlier period, yet tended to be pretty civilized. If we move to the 19th Century, compare the nastiness of the Napoleonic Wars and the American Civil War (conflicts of ideologies) to that of the Mexican, Austro-Prussian, Franco-Prussian, and Spanish-American Wars to name just a few examples of territorial land grabs/wars of hegemony.