Place the rump roast in the crock pot. In a bowl, mix together the broth, Italian dressing mix, au jus mix, and bouillon cube until dissolved. Pour over the roast. Cover the crock pot and cook on low heat for 6-8 hours or until the roast registers 130-140 degrees F on a meat thermometer (medium-rare to medium). Remove the roast from the crock pot and let sit for 5-10 minutes on a cutting board before slicing. While the roast is resting, if desired transfer the liquid from the crock pot to a saucepan. Bring to a boil over medium-high heat and let the liquid cook down. Serve with the roast. You can also thicken the liquid with cornstarch if a more gravy-like consistency is desired.