Microwave potatoes with 2 tablespoons water, covered, on high until potatoes just begin to soften, 7-10 minutes, stirring occasionally. Transfer to a 6-qt. slow cooker; add carrots. Meanwhile, mix 6 tablespoons softened butter with horseradish, garlic and seasonings; refrigerate half of butter mixture., In a Dutch oven, heat remaining 2 tablespoons plain butter over medium heat. Brown roast on all sides; transfer to slow cooker. Add broth and wine to Dutch oven; cook 2-3 minutes, stirring to loosen browned bits from pan. Pour over meat. Spread butter mixture over roast; cook, covered, on low until beef is tender, 8-10 hours., Remove roast and vegetables to a serving platter; tent with foil. Mix flour and remaining water until smooth; gradually stir into cooking juices. Cook, covered, on high until thickened, 15-20 minutes., Cut roast into thick slices. Top with remaining butter mixture; drizzle with 1 cup gravy. Serve with potatoes, carrots and remaining gravy.