I'm dumb. Scratch my question, I think I've figured it out. Spent too much time going "wtf" and not enough time thinking about how an opamp/comparator works
Please fill in any non-sensical BS with correctness.
The low voltage is not the primary issue, but instead the tremendous p-p (virtually AC) variation of V+. At low input voltages (ie 1V), there's ample voltage throughout the V+ curve for the opamp to work as it should and output whatever gain level (range) is selected. However, when you throw 10V on the input, the output of the opamp is saturated at all V+ values less than what is needed for the output to match the input gain.
Recall that 10V was stable on the 100V range. If my theory is correct, then increasing the input voltage on the 100V range to lets say, 20 or 30V, should elicit the same behavior exhibited by the 10V on the 10V range. OOH a chance to use the 6920B!
So this makes sense in my head, but then I went and played with buttons and confused myself. At 10V, I flipped the polarity of the EDC 501J and lo, -10.0000V--steady as could be. So why on earth would it work for a negative input, but not a positive?