How do I make dense firm meatballs

I would say don't use egg whites only yolks

Fiona, never tried that, but I'll try. My wife is the sauce guru, but her fam recipe calls for plenty of bread crumbs to firm them up with a good 80/20 ground beef. Getting a good sear before adding to the sauce with an 1/8 in of olive oil in the bottom of the pan.

I like to soak day old bread that has been hand crumbed in small amount of milk then give a slight squeeze after 20 min to remove excess fluid from now plumpy bread.

Keep the breadcrumb proportion relatively low yet add enough egg to keep the whole shebang together.