Thinking about it, unless they decide to push Pharos Sirius into the ilvl70 difficulty, or make it the final boss theme for Siren (as a Wandering Minstrel fight), the song would never be able to play fully, due to the short distance between bosses (if DPS is incredible).

Or they remove the mini-boss themes and just have the new Pharos Sirius theme play from start to Siren.