Lightly whisk two large eggs in a small bowl. In a separate small bowl, combine the breadcrumbs, Parmesan, garlic powder, onion powder, Italian seasoning, salt, and pepper.
Add the Italian sausage and ground beef to a large bowl with the whisked egg, breadcrumb mixture, and milk Use your hands to mix the ingredients together. Avoid overmixing the meat.
Divide and shape the meat into 24 meatballs, about 3 Tbps each. The easiest way to do this is to first divide the meat mixture into four equal portions, then divide each of those in half to make eight equal portions. Finally, divide each of the 8 portions of meat into thirds and shape them into balls to make 24 meatballs.
Place half of the meatballs in the bottom of a slow cooker in a single layer and cover with half of the marinara sauce. Add the rest of the meatballs then top with the remaining sauce.
Cook the meatballs on high for three hours or low for six hours.
After cooking, use a meat thermometer to make sure the internal temperature of the meatballs has reached 165°F. Give the meatballs and sauce a good stir to remix the sauce (there will be some fat pooling on the surface).
Preheat the oven to 350°F. Place the buns on a baking sheet or in a baking dish, then pile three meatballs into each bun. Spoon some of the extra sauce over each one. Top each sub with a little shredded cheese.
Bake the meatball subs in the preheated oven for about 5 minutes, or until the cheese is melted and the edges of the buns are toasty. Enjoy hot!