The dial position, ie -20dB for reference level, is arbitrary. It depends on your power amps gain, speaker sensitivity, listening distance, room size, etc.

In my setup, my 976 reference level is at -10dB on the volume dial because I set my amps gain lower.

For a given power supply, a balanced output have the potential to produce 6dB higher maximum level, because of the differential nature of the balanced system. However, their actual gain may be set by the manufacturer to produce the same nominal level than the unbalanced outputs at a given volume position. Or not!