Yup. The difference between your weight @ 0mph and the weight of the car @ 30, 40, 50mph acting on those springs is so different it's not worth comparing. Ive driven on stiff springs and bad dampers and they'll still bottom out in a heartbeat in certain conditions. This is why having properly functioning dampers has more to do with ride quality than spring rate does.So they can go from barely moving to bottoming out?
I cant picture how broken k-frame mounts would do this and not have other, worse symptoms.. have you seen broken k-frame mounts?The bang is probably bad strut mounts. Or it could be broken k-frame mounts.