There are no issues involving the RX480 series. Since you clearly state that from day one you could only run the card if there was a HD6570 in slot 2, your issues lies somewhere else.
I was up till 5am coz I can't sleep if I have a problem. Any other card will work in that slot but not the 480. The PCI slots look distorted and twisted (I do screw in my cards), and I don't remember if it was like that before. so my guess is because the 480 needs to draw more power some of the pins aren't connecting. Though it's still strange that it works when another card is in slot 2. Oh well I've move it to the x8 slot and it works. Oh well new, board new CPU.