Position racks in the middle and upper third of the oven; preheat to 400 degrees F. Scrub the potatoes and dry well. Bake directly on the middle oven rack until easily pierced with a knife, 45 to 50 minutes. Remove from the oven; let cool 5 minutes.
One at a time, hold each potato with a kitchen towel and halve lengthwise. Scoop the flesh into a large bowl, leaving a 1/4-inch-thick shell. Discard 2 of the potato skins. Mash the flesh with 6 tablespoons butter and 3/4 cup sour cream until smooth. Stir in the chives, parsley and 2 to 3 tablespoons horseradish; season with salt and pepper. Melt the remaining 3 tablespoons butter; brush the potato skins with 1 tablespoon melted butter and season the insides with salt and pepper. Set on a baking sheet.
Mound the filling into the potato skins. Return to the oven on the top rack and bake until the filling starts browning and the skins are crisp, 18 to 20 minutes. Remove from the oven and drizzle with the remaining 2 tablespoons melted butter.
Combine the remaining 1/4 cup sour cream and 2 tablespoons horseradish in a small bowl. Top the potatoes with the horseradish cream, bacon and more chives.