Preheat the oven to 250° and position racks in the upper and middle thirds. Toss the popcorn with the roasted peanuts in a large heatproof bowl.
In a large saucepan, combine the brown sugar with the butter, agave nectar, corn syrup and salt and bring to a boil, stirring until the sugar is completely dissolved. Boil over moderate heat for 4 minutes. Remove from the heat. Using a long spoon, stir in the baking soda and tequila; the syrup will foam. Immediately pour the hot syrup over the popcorn and peanuts and, using 2 spoons, toss to coat thoroughly.
Spread the popcorn on 2 nonstick baking sheets and bake for about 1 hour, stirring occasionally and switching the sheets halfway through, until golden and nearly dry. Turn off the oven, open the oven door completely and let the popcorn cool completely before serving.