Holy crap. I thought I had non-single ended drive, your post caused me to look again, and the diagram I have for my Hurco drives clearly shows it as single ended, but the colors on the diagram don't match the wires I have, so I guess I just figured it was a differential, not a signal and gnd. I switched them, and I clearly was sending AOUT to ground. Everything works as it should now. Thank you. I'm very glad MESA made their cards at least somewhat idiot-proof, I know many other ways I could have let the smoke out of a chip.
I'm going to owe a lot of people pints by the time I've got this thing running. I only hope I can be helpful to others at some point.