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.
I had to turn down the bias on one of my tube amps, it was going through tubes too fast. I didn't change it that much, around 10% I think. I could not notice a difference in sound. 
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I would think that 70% bias that you suggested is too low to get the intended sound that the manufacturer intended you to get. Yes, your output tubes may last longer, but what good will that do if it does not sound better that way? Listen to a familiar album with the tubes biased at the suggested setting, then back them off 10% or so and listen again. It should sound fairly similar. Those output tubes in your Lamm amp put out a tremendous amount of heat, even if underbiased quite a bit. I can understand that you want your tubes to last longer, but don't let the music suffer! 
Thanks for your replies but let's make one thing clear! If 31.5 is the correct bias and if I set the tubes only to 26.5 then is this under bias or over bias?