In a large pot, bring the reserved shrimp shells and seafood stock to a boil over medium-high heat. Reduce the heat and simmer to blend the flavors, uncovered, for 20 minutes, stirring and skimming the surface occasionally. Strain the broth into large bowl, pressing on the solids with the back of a spoon to release as much liquid as possible. Return the broth to the pot and keep warm.
Meanwhile in a medium, heavy saute pan, heat the olive oil over medium-high heat. Add the onions and cook until translucent, about 5 minutes. Add the carrot, celery, and bell pepper and cook until fragrant, about 7 minutes. Add the tomato and chile de arbol and cook for 5 minutes longer. Mix in the tomato paste and dried shrimp powder and cook for 3 minutes.
Add the cooked vegetables to the warm broth and stir. Cook on medium-low heat, covered, for 1 1/2 hours. Add the shrimp to the simmering broth. Turn off the heat and let the soup stand for 3 minutes until the shrimp are translucent and cooked through.