After much experimentation, I have discovered why the spectrum is alway flat at low frequencies. I thought I could narrow the bandwidth setting in order to narrow in on the low end; but apparently this limits the number of samples and allows frequencies above my selected bandwidth to be aliased into the narrow range, causing a display of, say, 390 Hz to be a summation of the ranges 0-390, 391-780, 781-1170, etc. Big problem. If I put in a filter to eliminate everything above 10kHz, then I can select a bandwidth above 10kHz and get a good display, although I can still not narrow in on 100Hz as well as I would like.
The solution would be to select oversampling to raise the frequency threshold at which aliasing will occur. According to the manual, there is a "max oversampling" option in the same dialog as the ETS oversampling rate. I can find the ETS oversampling box, but there is no "max oversampling". Where did that go?