Revelation: 37-mile ride three days ago, butt sore, 81-mile ride yesterday, butt not sore. There was no change in the bicycle at all, the only difference was better bike shorts!
So, I've been experimenting with saddle position, saddle orientation, and handlebar position, and drawing conclusions. But because the shorts that I happened to wear on a given ride was random, those conclusions were worthless.
It seems that I was confusing pain on the surface (skin) with deep down pain.
On the butt-sore ride, I was wearing
these. They have a simple chamois. On the butt-comfortable ride, I was wearing
these. Although inexpensive, they have a thicker molded pad, and came highly recommended from someone on this forum. I also wore some non-padded tights on the outside of the shorts. I was also wearing suspenders (poor-man's bibs).
Based on this, I'm going to shell out some dough for some more-expensive shorts.