@olivierlambert
Sorry about the delay, got a lot going on.
Anyway, was able to pick this up again and here's what happened this time. Booted XCP, noticed there were a bunch more updates, so ran the update so I'm collecting information from the very latest and greatest.
Re-boot XCP and start the TrueNAS VM with NO passthrough devices. As expected, that started up fine. Stopped TrueNAS and added all the devices and started TrueNAS again. This immediately caused the server to reboot itself. Hmmmmm.
On the restart of XCP-ng I collected the output from "xen-bugtool --yestoall" and also the /var/crash directory (how do I upload a tgz), which hopefully will give a clue as to what's going on.
I also have the output from "xl pci-assignable-list" and "xe vm-list params=other-config uuid=<uuid>" showing the passthrough devices if needed.
Cheers.