Using the pressure cooker is by far the best way. Why would you want to take 10 times longer to cook them in a slow cooker when you've got a fast cooker? That why pressure cookers are essential cookware at high altitudes, since they completely negate the low air pressure that dramatically reduces the cooking temperature. Some beans will never soften without a pressure cooker because regular cooking doesn't reach a high enough temperature to allow the proteins and starches to bind to the water.