I had an LGA1155 motherboard (MSI P67A-GD53) and I purchased a new 390 the other day, which had the exact same issue. I had to update the bios (which unfortunately bricked my motherboard), meaning I had to buy a new mobo and cpu which arrived on thursday.
Turns out it was the motherboard after all as my 390 is now outputting a signal to the monitor and working perfect.
Its not unusual that motherboards need a bios update to accept a new graphics card.
What prompted you to update the bios was it some mention in the release notes ?