I disconnected the first 4 XLR’s from the Mac leaving just 1 XLR between Mac and amp connected and buzz is gone???
Very interesting. OK, let’s stay on this path for a moment. From my understanding, and just so I’m 100% sure here, you’re saying with one XLR cable connected, there’s no buzz in any of the speakers, yes? If that’s the case, I would also try using that exact same cable, and plug it into all of the inputs and outputs you use when connecting the Mac to the Sim. When doing this, does this produce the same result(s) - no buzz? I would also recommend trying that same scenario with each cable, one at a time (I know this is probably a lot of work, but what we’re doing is seeing if all cables produce the same result). It might not be a bad idea to help identify which cables are which. Maybe a sticky-note (or something like it) could temporarily be affixed to each cable for identification purposes.
I’m also wondering if as each XLR cable is reconnected (when using all of them), does the buzz increase a little bit more in volume level as each cable is plugged in (or when the buzz is heard, it always remains at one volume level)? Also, is it safe to say that when the buzz is heard again, as each cable is plugged in, the buzz is directly attributed to the corresponding channel and speaker only?
Sorry for all of the questions, but this is the only way I can diagnose this, without physically being there.