In a large pot, bring 1 gallon of water and cubed potatoes to a boil and cook until tender. Drain and set aside.
In a saute pan, melt 3 tablespoons unsalted butter over medium-high heat. Add onion and celery, season with salt and pepper and sauté until translucent, about 8 minutes. Set aside.
In a large, heavy bottom stock pot, melt 8 tablespoons of butter over low heat. Add flour, stirring until butter and flour are combined and there are no lumps.
Very gradually add milk, starting with ½ a cup at a time, continually whisking so there are no lumps.
Dilute 2 teaspoons of clam base in ½ cup hot water, add to milk base.
Add sautéed vegetables, scallops, shrimp, clams, and crab to milk base. Add half and half and any remaining milk. Add potatoes, stir to combine. Season with kosher salt and black pepper.
Bring chowder to a low simmer over low heat, for 1 hour.