Rinse porcini mushrooms and place in a small bowl with 2 cups hot water. Soak until soft, about 20 minutes. Rub porcini to remove any grit; lift out and squeeze liquid into soaking water. Coarsely chop porcini; reserve soaking liquid.
Place hazelnuts in a 9-inch pie pan and bake in a 350° regular or convection oven, shaking pan occasionally, until nuts are golden under skins, 10 to 15 minutes. Pour into a clean towel and rub to remove as much skin as possible. Lift nuts from towel, leaving skins behind; coarsely chop nuts.
Place half the bread in each of two 12- by 17-inch baking pans. Bake in a 350° regular or convection oven, stirring once, until lightly browned, 12 to 15 minutes. Pour into a large bowl.
Trim off and discard dark green tops and root ends from leeks. Cut white stalk in half lengthwise and rinse well, flipping layers to release grit. Coarsely chop. Rinse cremini and shiitake mushrooms; cut off and discard shiitake stems. Thinly slice both types of mushrooms.
In a 5- to 6-quart pan over medium-high heat, melt butter. Add shallots, leeks, and cremini and shiitake mushrooms; stir often until mushrooms begin to brown, about 15 minutes. Mix porcini, nuts, celery, parsley, thyme, and sage into mushroom mixture. Stir into bread and add salt and pepper to taste. In a small bowl, beat together eggs, 1/2 cup of the reserved porcini mushroom-soaking liquid (pour in carefully, leaving grit behind), and 1/2 cup chicken broth. Pour over bread mixture and mix well. If you prefer a moister texture, add up to 1/2 cup more porcini mushroom-soaking liquid or broth. Pour into a shallow 3- to 3 1/2-quart baking dish. Cover with a buttered sheet of foil.
Bake dressing in a 325° regular or convection oven until hot in the center, 45 minutes to 1 hour; uncover and bake until top is golden, 10 to 20 minutes longer.