The Meatball Marinara is the rare line-built sub whose filling arrives already wet, and that single fact dictates the entire build. Where a cold cut sub is dry components assembled to order, this one starts with meatballs held in hot marinara in a well, scooped onto the roll sauce and all. The structural problem is no longer how to layer dry slices but how to get a saturating sauce into soft bread and still wrap it before the crumb gives way. Everything about the assembly is a response to that, not to flavor.
The craft is in the sequence and the speed. The meatballs are pre-cooked and held submerged so they stay moist and portion to a consistent count rather than being formed or cooked to order, which keeps the build to a single scoop. They are laid in a row down the split roll so the sauce distributes along the length instead of pooling at one end, and cheese goes directly on the hot meatballs so it slackens against them and forms a partial barrier between the marinara and the upper crumb. The roll is the same soft line bread used across the format, and here its softness is a liability the build has to manage: the sandwich is assembled fast and wrapped immediately, because a marinara load left to sit will work through a tender crumb and the structure depends on the bread not having time to surrender.
The variations are bounded by the wet-filling logic. Toasting the assembled sub briefly sets the outer crumb and helps it stand up to the sauce, which is less a flavor choice than a structural one. Adding more cheese, a second scoop, or finishing with grated hard cheese stays inside the same single-well build. The cold meat subs that run dry components on the identical roll are a different assembly problem entirely, and they deserve their own articles rather than being crowded in here.