What would happen when you under Bias your tubes?

Hi, I am a tech noob and I always wanted to ask if it is safe to under biasing tubes and especially the power tubes? Will that affect performance or will that do damage to the amp? I am asking because my power amp which is the Lamm ML 2.1 has manual bias and when I left the bias unchecked for awhile. It somethings jumps much too high then originally set and those 6c33c doesn't last! I mainly set the bias to 70%. Is that OK? Thanks.