Rebound and Soulfire Grand Master's ability are both replacement effects trying to replace the same event (spell going to graveyard as it resolves), so you get to choose which one to apply.
Magic Rules Tips
…and knowing is half the battlefield
…and knowing is half the battlefield
Rebound and Soulfire Grand Master's ability are both replacement effects trying to replace the same event (spell going to graveyard as it resolves), so you get to choose which one to apply.