No idea about service option in S Fl but a couple more questions. Do you get anything out of the headphone jack? Do you have any output cards installed or can borrow to see if you get output from those? It’s entirely possible that the DC rails on the Omni board(s) are locally regulated and you simply have a regulator IC going bad. That’s probably an analog world POV, I suspect that even if such a thing is the case it’s still a PC board swap.
It's almost certainly a board swap. The M7CL power supply just provides something like +20vdc - that goes into a big board under the center section which turns that single voltage into every other voltage needed by the various sections of the mixer.
It's sometimes useful to check to see if any of the digital outputs work, such as the AES output or one of the card slots. But really it's likely that one big card with a bad section feeding the analog outputs. You can measure voltages at the heavier ribbon cables which carry power; there's often even a legend by the connectors.
