Thanks very much. That was exactly my problem; setting DepthStencilState solved the issue.
I didn’t realize the utility of CullClockwise and CullCounterclockwise for avoiding the unnecessary work of drawing the far side of the object and I will certainly manipulate the vertex order to take advantage of that!
Excellent reminder about minimizing extra work by declaring at the class level instead of the method level where appropriate, too.
Thanks very much for your extremely helpful response!