My earlier intended question has been resolved.
I was asking a question about driving a subwoofer from the Emotiva XDA-3 DAC.
I was trying to run the sub with a single RCA cable.
I was previously driving the sub from an earlier device with single LFE RCA.
I have been able to resolve this error by using two RCA cables, right and left channels. This apparently turns something on that is needed between the Emotiva XDA-3 and the subwoofer.