Yes, Briess extracts do have high sodium due to their local water company using ion-exchange to soften the municipal supply. If you were adding only a minor addition (30 to 50 points of gravity) then it shouldn't really be a problem. It could be a problem if you were making a big beer with mostly extract.
While alkalinity can produce problems with pH and subsequent tannin and silicate extraction, its not the only way to get them. As you might expect, I am fairly particular about water chemistry. But I was producing a mild tannin expression in my first few beers when I switched over to my new brewing equipment. It turned out that I was oversparging and introducing tannins in that way. I had been stopping the runoff at 2 Brix, but that was apparently too low. I now stop at around 3 Brix. That solved the problem.
Joe, I note that you are in north Chicago. For some of Chicagoland, the water is from the Great Lakes and other places are groundwater. I'm curious about your source. The lake water quality is relatively constant. The lake alkalinity is modest, but should still have some neutralization for some styles.
The Great Lakes Compact has changed what can be done with lake water. If your wastewater does not make it back into the lake, then I understand that they are requiring those utilities to stop using lake water and find another source so that this use isn't draining the lake. If your utility had to get a new source, that could be a source of your problem. Other sources in the region are typically much more alkaline. Hopefully you have been neutralizing your brewing water with acid, as needed. That should remove that source of tannin and silicate from your beer. But it won't solve oversparging.