It is always best to start with a light tonearm. It is easy to add mass, much harder to take it away. You want to get your resonance point between 8 and 10 Hz for the best bass. When the cartridge gets to the resonance frequency you will see it start to bounce. You simply add mass until you get the resonance frequency down where you want it. There are head shell weights and Soundsmith sells a set of graduated head shell screws that are brilliant.
Effect. If the resonance frequency is too low the tonearm can bounce into the air when it hits a warp. Too high and your bass performance declines and you invite acoustic feedback. When you turn the volume up the system starts to howl.