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 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?
Luna, the bias is supposed to be set so that the tube is being used in the middle of its operating range, in a region that is the most nearly linear for input-output tracking. Bias it too low or too high and it becomes increasing non-linear at either end of the operating range, respectively.

The actual value of bias depends on the type of tube. If you can refer to a tube manual, that may tell you the recommended bias. Or you can ask someone here who has a tube manual, listing the tubes you wish to have looked-up.
follow the manufacturer’s direction, try to stay within 10% low or high, 5% even better
The bias voltage is set to have a class A amplifier operating in the linear portion of the range.  The amplification across the frequency range will only be linear over s relatively small portion of the tube's amplification range.  You can easily go out of class A operation and introduce non-linearity to your signal thus providing sound that was not in the original audio source.

(Old Navy Engineer from vacuum tube days)