Preheat the oven to 350°F (175°C).
Carefully hollow out the center of the round loaf of bread, leaving about 1-inch thick walls for support.
In a large mixing bowl, combine cream cheese, sour cream, mayonnaise, garlic powder, onion powder, salt, and pepper. Mix until smooth.
Add chopped spinach, cheddar cheese, and Parmesan cheese to the mixture. Stir well until all ingredients are fully combined.
Pour the spinach dip mixture into the hollowed-out bread bowl and place it on a baking sheet.
Cover the bread bowl lightly with aluminum foil to prevent excessive browning, if desired.
Bake in the preheated oven for 30 minutes or until the dip is heated through and the bread is slightly toasted.
Remove from the oven and let cool for a few minutes before serving.
Serve with the removed bread pieces for dipping, and fresh vegetable sticks or crackers if desired.