As i see, you forgot to disable CPU graphics!
Disable it in a BIOS
Also, if CPU graphics set to "Auto", so use x16-x16 riser in a first slot (meet same at my Asrock Anniversary)
I'm use the cheapest non-powered one case MB supports one card freely

P.S. Asrock noted that in multi GPU system user need to install first card in x16 slot and only than install other cards (may be not need, but i'm follow a note)