In a large bowl, combine the ground beef, ground pork, breadcrumbs, eggs, parsley, onion, garlic, Parmesan cheese, oregano, salt, chili powder, and black pepper. Fold the ingredients together with clean hands until fully incorporated. Using a tablespoon-sized scoop, form the mixture into equal-sized meatballs. Set aside.
To the bowl of your slow cooker, add 1 ¼ cups of the marinara sauce. Place the meatballs on top and pour the remaining ¾ cup of the marinara sauce on top. Slow cook the meatballs for 4 hours on low or 2 hours on high. Keep them warm while you prepare your queso.
In a large, heavy-bottomed saucepan, melt the butter over medium heat. Whisk in the flour and cook for about 2 minutes, whisking constantly until golden brown. Whisk in the milk and cook for about 2 minutes or until thickened. If you find that the mixture is too thick (paste-like), add more milk until the consistency is just thick enough to coat the back of a spoon. Reduce the heat to low, and add the Parmesan and white cheddar cheeses in ¼ cup increments. Make sure each addition is fully melted before adding the next. Stir in the cayenne pepper. Keep the queso warm on the stove until you are ready to assemble your sandwiches.
Assemble your meatball subs by layering the meatballs on a soft Cobblestone Bread Co. White Sub Roll. Drizzle with queso and sprinkle with fresh parsley. Serve the meatball subs immediately or alternatively, you may wrap the subs in foil and refrigerate. When you are ready to serve, preheat the oven to 400 degrees F and place the subs in the oven for about 10 minutes or until warm.