When you run the tubes at a higher bias than specified for a given load, essentually you are over dissapating the tube which runs down the plate and cathode faster than normal. There are a number of people who successfully run certain tubes past their rated dissapation but this is territory for experienced tube amp builders with experience and testing equipment for how to do so. Chinese 300B's apparently need not apply to this role.
When you run the tubes lower than designed for, you will probably lengthen life of the tubes but you have also changed the operating points and there is no gaurantee that you are running the tubes in their optimal sounding settings, unless you know how to read curves and set your own settings. In this case though, there isn't an concern really for shortening the life of the tube.
