The use of 6870 in a 1,1 means NO BOOT SCREEN since there is no EFI32 EFI for them.
So, this could just mean that something is stopping the boot process before it gets to desktop.
You should have an EFI card on hand for times like this.
An old 2600XT would be ideal, works in all Mac Pros and doesn't need power plugs.
Ot use "screen sharing" to log in from another Mac and see hwat is going on.
Perhaps one of your power plugs has gone bad?