How to get Kerberos working against a named SQL Server instance in Power BI Report Server?
After hours of troubleshooting, I've got this working. I had to allow the PBI service account to pass credentials to the destination SQL Server(s) on an EXPLICIT port, even when connecting to a default instance on 1433. If anyone has any input, feel free to comment, but here's what worked for me:


Open Active Directory Users and Computers
Right-click the service account running Power BI Report Service, and click "Properties"
Click the "Delegation" tab, then "Add", then "Users or Computers"
Enter the service account of the DESTINATION SQL Server instance and click "OK"
Click the MSSQLSVC Service that shows a port, whether it's the default 1433 or another port. If you have multiple named instances, add the entry for each port number.
Click OK, then OK again.
Restart the Power BI Report Server MACHINE, not just the service.
Open a Power BI (.pbix) file that has uses DirectQuery against a different SQL Server instance.
Credentials should now be passing from you (the client), to Power BI Report Server, and then on to any destination server(s).