Perhaps this is me being cynical, but the federal system had been dashed upon a rock in 1861. Though between then and 1913 the election of senators through state legislatures continued to give states a more or less direct means of pursuing their interests in the national government, it was by that time more playacting. It was a short time after 1913 that our limited Republic went down the scrapper.
That may have had something to do with the democratization of the senate or the creation of the federal reserve and income tax amendment which you mentioned. It could have been the triumph of progressivism in the crises (which they never let go to waste) of WWI and the Great Depression. Certainly the switch in time that saved nine contributed, horribly. I don’t like to see it all as Civil War hangover, but that to me is the great turning point, not the 17th amendment.
Speaking of the Triumph of Progressivism, there is a documentary available to view on-line; Agenda: Grinding Down America. It’s 90 minutes, well worth the time: http://vimeo.com/52009124