I freeze orange juice in muffin pans so the sangria won't get watered down from ice. I also used frozen concentrated orange juice made with 7-Up instead of water because I like fizzy stuff. In that case, add the frozen concentrate, but don't add the 7-Up until just before serving.
If it's too potent, try adding something sparkly like club soda or 7-Up to each glass. If it's not potent enough, use Vodka to make the orange juice.