Just wondering...am I correct in understanding that when all XLR’s are unplugged between the Sim and Mac, there is no buzzing? If that is the case, if you reconnect just 1 of the 5 XLR cables, does the buzzing return? If so, in all speakers?
Just for the heck of it, I would also trying connecting the grounds on the rear, between the the Sim and Mac (with any piece of wire you have laying around), and see if that changes anything. Try this with and without the XLR's connected.