Blank Boot Screen but the Mac still login successfully.

I replaced the GPU (many thanks to the well done ifixit.com tutorials) and the Logic Board Battery after a very long inactivity period.

I made a a clean OS X installation (El Capitan) and now both my iMac and the new GPU work well after the user login, anyway I cannot see any video signal before the user login screen.

All the 4 diagnostic LED are working I checked, but I can’t get the HD selection (option command) at the machine startup - or maybe I can but I cannot see it.

I'm baffled, what is that strange behavior due to?