Even pushing the sun strength up to 600 doesn't change how bright objects in the scene are. However I notice that when I made a test file to post with this question, the sun was as bright as expected.
All I did when making that file was delete all objects except for the sun, and one other big enough to indicate how the light was behaving. I created a new camera to look at it, so I thought maybe somehow the camera is the problem, but in the original file, a new camera made no difference.
Here is a screenshot showing all controls I know of that might be involved, and everything seems okay. Besides, the other file, which shows up on the right in another window, has all the same settings. It would seem this has to do with other objects, however I can't figure it out.
How can I get this scene to respond to the sun?