I attempted to revert the changes made, starting with the BIOS. I downgraded directly from version 4.60 to 3.20 of the BIOS.
The BIOS downgrade did it. The system booted with Hyper-V enabled following the BIOS downgrade.
I am unsure why this worked and why the BIOS upgrade affected virtualization, but having Google'd for several hours without finding the solution, I thought I would post it here in the hope that my discovery could help someone else who encountered the same issue.