I believe King Juan Carlos has never been called, or has never been given the title of, Prince of Asturias. The last Prince of Asturias was Prince Alfonso, son of Alfonso XIII. Juan Carlos never had the title due to the course of events leading to the exile of his family, and with his designation as "Prince of Spain" (not "of Asturias") and as next king by Franco, instead of inheriting it.
So Queen Sofia has never been Princess of Asturias.
While on the subject of the SRF, I remember seeing a photograph of the invitation for the confirmation of Elena when she was young and she was listed as "Princess Elena of Spain and Greece" in Spanish (Espana y Grecia)