Final update:
Well, this was a very interesting exercise.
I realized, after trying various Library backups and getting corrupted info from all the ones I tried, that my scheduled Macrium full image backup had just run on Saturday (and as I'd said, everything was fine then).
I restored the entire partition from the Macrium backup, rebooted, and voila, everything was fine with JRiver again. Everything was exactly as I remembered it should be.
So I tried another experiment. I then did a library restore from one of those library backups that had restored, but had seemingly contained corrupted equalizer presets (all sliders at +12). Library restore completed fine as before, and I checked the equalizer presets. They were all fine.
SO, the net out was that something was corrupted with MC itself, such that the program ran (seemingly without problem and without generating any error messages) but DSP settings were screwed up, but the corruption was so deep that DSP settings could not be properly restored from a Library backup. That such a thing could happen is troubling.
Obviously, performing the Macrium restore corrected whatever the underlying problem was, and thus revealed the true-good nature of the library backups themselves; it was the restore process that was problematic.
If there exists any sort of integrity check for MC (not just the library, but the executables, data files, and registry as well) that could identify a problem or corruption (similar to SFC for Windows) I'd love to know about it. Does anyone know of a way of diagnosing this sort of problem?
Without Macrium I'd have been up a creek. As is stands now, everything's back to working fine again. For anyone with whom this strikes a chord, I'd highly recommend Macrium Reflect. Great program.