Well, there's more tinkering needed, but the use of speaker level inputs made a significant improvement!!! I have no idea why -- maybe my preamp was a bottleneck? Anyway, I now have more bass all up in my face.
I DIYed some speaker cables from stuff I had lying around the basement. I could only listen for a few minutes because I have out of town guests who are running from Irma. In a few days, I'll do some more experimenting and report back. Nevertheless, I realize that I still need to perform those measurements.