I had some trouble when I changed the opensource driver nouveau to the proprietary nvidia-390.
The package is broken and when I try to repair It, I can't.
I've also tried to make the change back to the opensource driver anyway, but It became impossible!!
There is a curious observation to make: If I boot with the HDMI cable into the 750 Ti, there will be no video out. I have to boot ubuntu with the APU and then change to the graphics card.