Clean, peel and devein the shrimp saving the heads and shells.
Drizzle 1 tablespoon EVOO in a saucepot and toast the shrimp shells and heads. Add leek, bay leaves, peppercorns and a couple of slices of lemon. Cover with stock and add saffron threads. Bring to boil then reduce heat to a simmer. Reduce by half.
Bring a large pot of water to a boil for the pasta.
Heat a large pan over medium-high heat with 2 tablespoons of EVOO, 2 turns of the pan. Add shrimp and sear 1 minute on each side then remove from pan. Dress shrimp with lemon juice from remaining half lemon. Return pan to heat, add 2 tablespoons more EVOO, shallots and garlic, and swirl or stir 2 minutes. Add tomato paste, thyme, sugar and chili flakes or chili paste, stir a minute more and add dry vermouth or wine.
Strain saffron-shrimp stock and add to pan then swirl in butter to melt. Stir tomatoes into sauce and bring to a simmer. Add mussels, calamari and shrimp then cover and cook 7-8 minutes to open mussels and cook shrimp through.
While seafood cooks, cook linguine in salted boiling water to al dente, reserve 1/2 cup cooking water then drain.
Toss pasta with sauce and starchy cooking water, transfer to serving platter and serve.
