I used to be a Mackie service center. I don't recall ever seeing shorted outputs in those amps- at least very rarely. The problems I saw were ribbon cables- not the issue in your case. The other was shorted diodes- usually the 1N5404's off the rails. There was also a Mackie amp (maybe this one?- I don't recall) that had triacs on the output connector boards like the crowbar protection in Peavey's. Those triacs would sometimes short, but rarely. You can tell if the triacs are shorted simply by measuring across the output binding posts. I'd start by checking those larger 3A diodes.