Just a theory question here.
If pro amps need 1.3V to deliver max power and the input is 1.3V unbalanced, the power amp gets 1.3V. If the mixer output is balanced that is + - 1.3V so the preamp will subtract in the input stage and give 2.6V to the power amp.
Or is balanced analogue output not + - 1.3V at 0VU
Cheers
GCT