TL;DR: The computer freezes whenever GPU pass-through is used
The virtual machine works fine booted without GPU pass-through, but when I firstly booted using GPU pass-through my screen had no output so I needed to force shutdown. I then tried changing the graphics adapter from from GPU to integrated graphics, which meant I needed to switch the cable from my GPU to my motherboard. When that launched the VM then the host machine froze, but I could still see the Ubuntu screen (and audio kept playing, although it lagged a little).
My PC contains an AMD R9 390, an i54690K (running onboard graphics) and Ubuntu 18.04 LTS. When I ran all the checks it said they supported GPU pass-through. I also run dual boot with QubesOS (mentioning because I am out of ideas what else could cause the problem).
I have one monitor, and currently 1 DVI dual link cable (however I can get an HDMI cable) I do not have a second monitor/tv.