I know this is a ways back, but I'm pretty sure the Savior was powered by 'Sparda's blood', ie. Nero, and of course with Sparda's power in the form of his sword. Even then, without Sparda, Kyrie, or Nero, what remained of Sanctus was enough to power it to some extent.
It couldn't have been Yamato, since it was almost immediately given to Agnus upon retrieval to open the true Hellgate, and remained there until Dante reclaimed it. The only way it could have been Yamato is through some initial act that made it no longer useful, ie. stabbing Nero's arm to get to Sparda's blood. Even if that were the case, he wouldn't have needed Yamato as it was only a means to an end, not the primary source of power.