Since I did not find a schematic or function sheme for the outputs the question is whether the load on the + output will also incluence the - output ? Since the outputs are not really floating and referenct to “ground” I assume that there is no cross connecion. Some other audio hardware balanced output may have such a cross connection which has influence from one pole to the other. How is it managed at the QA403 ?
AFAIK, the outputs are derived from the differential outputs from the D/A converter, buffered, amplified and attenuated (relays) using several opamps to give the output range we have. I would expect the load on a positive output would not affect the negative output, unless it was so severe as to cause supply fluctuation.
The output impedance of the QA403 is ~100R per leg SE, so 200R for differential.