Put crab shells, bay leaves, and crab-poaching liquid in a medium pot. Bring to a simmer, covered, and simmer, stirring occasionally, until broth has a gentle but distinctive crab flavor, about 1 hour. Strain broth through a fine-mesh strainer. If using crab-poaching liquid, return to pot and boil, uncovered, until reduced to 2 cups, 10 to 15 minutes. Set aside.
In another medium pot, heat oil over low heat. Add onion, celery, and bell pepper and cook, covered, stirring often, until very soft and translucent but not browned, about 15 minutes.
Add wine and tomato paste to vegetables and cook over medium heat, uncovered, until wine has reduced by half, about 10 minutes. Add 2 cups cream and the crab broth and simmer over low heat 15 minutes, uncovered, to meld flavors and thicken a bit.
Put six small, shallow soup bowls in a 200° oven to warm. Whip remaining 2 tbsp. cream into soft peaks.
Strain soup through a fine-mesh strainer into a large bowl and stir in butter and salt. Ladle about 2/3 cup into each soup bowl. Top with a mound of crab, if using, a small spoonful of whipped cream, and a sprinkle of herbs.
*Call your fish shop a few days ahead for crab shells, as they're not always available.