I spent an ungodly amount of time trying to get this VM to work, so here's what I've done to hopefully streamline the process for others using 6000 series Radeon GPUs.
1. Disable SAM/Resizable Bar in BIOS (this was the crucial step for me), I also disabled above 4G encoding, but I'm unsure of the effects so far
2. I have had no issues installing the latest AMD GPU drivers, so I assume that these are safe to install on the guest system
3. Only use Q35 in the BIOS because it has better support for pcie passthrough devices. (which AMD GPUs require)
4. Dump the bios from your card and set it as the vbios in the VM creation page if your AMD GPU is your vga-out card
5. Set your primary sound card to the GPU sound card
That should leave you all set! If anyone else has anything else they feel should be added, please reply and I'll update this topic!
Thanks @ghost82