So are you using ASUS graphics drivers for AMD that are listed? I would take out the 7750 and un-install AMD drivers.
I would then install the drivers from ASUS. I would shutdown and then install the 7750. Then move the cables for your monitor and hook them to your 7750 port.
Restart you system, and you should have only the discreate 7750 working with the onboard graphics being disabled. You can't do Dual graphics with a 7750.
If you still have no Windows Signon screen check to be sure that the BIOS is up todate.
AMD A10-5800K, XFX 7850 Ultra OC Edition 975MHz, 16GB RAM, Win 8.1 Pro 64 bit, MSI 7778 motherboard, 500W PSU, HP P7-1235, CCC 13.12 WHQL