I have my pc connected through hdmi to my denon avr. Previously I had a gtx 670 everything worked fine. Now I got a 970 and when I start my pc the video only appears when the windows login screen appears. So I cannot go to the bios or post screens. I have an x58 motherboard wonder if that's the reason.
My 970 did the same thing, along the HDMI, till I made sure I set CSM to Auto on my motherboard. After this, the video started to output on post, and not just when it got to the Windows Logon screen.
Since your motherboard is X58, it probably doesn't have a UEFI BIOS, which would mean you wouldn't be able to do the same thing. The issue itself
could be due to another reason; however, I didn't test further once I got it to output during post.