I have a loud hum when using rca out from dac10 to sub when my sta 9 are used in mono mode with balanced xlr cables, any help out there?
You might try simply running an additional RCA cable from the STA-9 to the DAC-10 and see if that helps.
This is an additional cable.
Just keep your system hooked up as now, and run that RCA cable to the RCA analog in. Might take only one, or maybe from both STA-9s
Do not change "any" switches or settings.
And before doing this, make absolutely sure you have the speaker cables set up EXACTLY as the diagram designates.