Arrange chiles in heavy large skillet over medium-high heat; toast on both sides until slightly blistered and puffed, pressing often with spatula, 3 to 4 minutes per side. Transfer chiles to medium bowl (reserve skillet). Cover chiles with hot water and soak until soft, about 45 minutes. Drain chiles.

Meanwhile, place reserved skillet over medium-high heat. Add onions, tomato, and garlic to skillet. Cook until slightly charred, turning often with tongs, about 10 minutes. Transfer to work surface. Coarsely chop onion and tomato; place in medium bowl. Reserve garlic.

Place strainer over large saucepan. Working in batches, puree chiles, prunes, onion, tomato, and reserved garlic in blender with broth, wine, and orange juice until smooth. Press puree through strainer into saucepan. discard solids in strainer. Simmer sauce in pan until reduced to 4 cups stirring often, 10 to 15 minutes. Season sauce to taste with salt and pepper. (Do ahead: can be made 4 days ahead. Cool, cover, and chill.)

Arrange pork in 13x9x2-inch glass baking dish. Pour soy sauce over. Turn to coat. Refrigerate at least 1 hour and up to 4 hours, turning occasionally.

Preheat oven to 375°. Remove pork from soy sauce; pat dry. Sprinkle pork all over with salt and pepper. Heat oil in large skillet. Sear on all sides until brown, about 8 minutes. Transfer to rimmed baking sheet. Repeat with remaining pork tenderloin.

Transfer ⅔ cup prune sauce to small bowl; reserve remaining sauce for serving. Brush pork with some sauce. Roast until thermometer inserted into center of pork registers 145° to 150°, turning occasionally and brushing with more sauce, about 25 minutes. Transfer to cutting board. Let rest 10 minutes.

Cut pork crosswise on slight diagonal into 1/2-inch-thick slices. Arrange on platter. Spoon some of reserved sauce over. Serve with remaining sauce.