My plants have had yellow and brown leaves on the base, spreading slowly to the top, for about three weeks. I researched, and saw early blight, and verticillium wilt as possibilities, but I'm not sure what I'm dealing with and how to treat it.

There are more distinct brown circles on some leaves now, but yet other plants just have non-circle brown splotches.

Here are photos of one plant:

And here is the plant next to it, without the same brown circular spots, but still with yellowing and brown edging on leaves: