mjhjmtu

I got a Windows installation onto KVM under Ubuntu Server 18.04, and now I want to pass through my EVGA 1070 GTX card to Windows. I followed the instructions here (except that I had already set up my Win 10 guest). I got to the end of part 4 of this tutorial, tried to restart and since then have not been able to boot.

With the monitor plugged into the GTX1070 (I did this at first by accident, but maybe it will help):
and then I restarted and froze during boot at the line:

vfio-pci 0000:43:00.0" vgaarb: changed VGA decodes: olddecodes=io+mem,decodes=io:mem:owns=io+mem

With the monitor plugged into the Radeon, I just get a black screen (not even the motherboard screen). But also no "no-signal" indicator. The power light on the monitor flashes as if the computer were asleep.

So it seemed obvious to me that the problem was with the changes I had made in step 4 (either to /etc/default/grub, /etc/modprobe.d/local.conf, or /etc/initramfs-tools/modules). So I just booted from an Ubuntu live USB so that I could access those files and change them back to the way they were. However, now I get kernel panics booting from the Ubuntu Live USB:

I don't see how it could be possible that changes in the root volume would prevent me from booting from a USB stick, and that same USB stick boots fine on a different computer.

So, does the first error give any clue as to what went wrong? Additionally, how could it have messed up booting from a USB? Why does the card I did not try to passthrough not give output when booting from the root drive?

AMD Threadreipper 1950x
x399 Taichi motherboard
MSI Radeon RX 470 (intended for host)
EVGA GTX 1070 (the one I am trying to pass through)
Host OS is Ubuntu Server 18.04