Place frozen meatballs and marinara sauce in slow cooker. Stir together until all meatballs are covered in sauce.
Cook on low for 4 hours or high for 2 hours. Once cooked through, they can remain on the warm setting for several hours until you are ready to serve.
Preheat oven to 400 degrees. Open sub rolls and place on large baking sheet.
Spread butter on the top portion of each sub roll and sprinkle with garlic salt.
Place cheese slices on bottom portion of each roll.
Bake rolls at 400 degrees for 10 minutes, or until bread is toasted and cheese is melted.
Fill rolls with meatballs and sauce. Serve immediately. Enjoy!