The quality of coilover can play an affect on this as well. Also, almost every coilover you buy in the aftermarket will come with a stiffer then factory coil spring. This is what can give you a more "rough" ride. Depending on type of coilover you have purchased you can sometimes contact the manufacturer and get "custom" springs for the coilover at almost whatever spring rate you want.
Something to keep in mind, the coilover you purchased will be valved for the springs that they would have come with. Changing the spring rate drastically will result in you needing to purchase a re-valved damper.