I don’t have a second rack unit - I’ve successfully built a couple of FET/500s previously. I got the active link so if I ever do build a second one it’s already be in place without cracking the unit open. This issue is super mysterious to me... Hopefully Mike or somebody sees this, otherwise I’ll send it into Hairball and see what they say.
Okay, I found a hole in my testing regime. The XLR Input +/- terminals ARE shorting to chassis ground when and only when where is a cable inserted and that cable is connected to any other device. I'm going to try removing the jack and making an XLR F pigtail cable to attach to the board, and see what I get.
EDIT: I tried that, and it made no difference, so the issue is definitely somewhere on the board that's beyond. Shipped it to Mike at Hairball today, and I absolutely cannot wait to find out what happened here.