How do you go about under volt cards? Can you do this though MSI afterburner, or do you need to use some custom Bios update for cards?
Just tick 'unlock voltage control' in AB compatibility properties, after that you can move core voltage slider in the opening screen.
not with nvidia
True, with nvidia you can't undervolt. Only way to save power is to lower TDP which actually works quite well on ethereum mining. My 5x970 can do 83MH@600w from the wall.
i'm reading now about an issue coming from the OS for six nvidia 970 or 980, is this true with amd also
and was it solved or still ongoing? what is the problem exactly?
With AMD cards, you can have up to 8 GPU in a system. You can change the voltage with MSI on those 8 GPU without problem.