What would happen when you under Bias your tubes?

no

not if you want proper sound the amp maker intended

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At the risk of buring tubes faster than he wishes. 
I tell my tech guy, bias  low, 
Richard beinga   tech perfectionist, does not like  biasing low, So we compromised after his *as specs biased* blew a  cap (Mundorf $50!!) + had to buy a  pair of tubes,, although only 1 tube went bad due to blown cap, . He said order  a  matched pair. 
My amp sounds perfectly fine biased a bit lower than specs require.

I mainly set the bias to 70%. Is that OK?

no

not if you want proper sound the amp maker intended

 I mainly set the bias to 70%. Is that OK?

Yes ALWAYS under bias. 
= Better tube life, less risk at blowing a  power tube. 
70 is OK

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)

follow the manufacturer’s direction, try to stay within 10% low or high, 5% even better

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.

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?

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! 

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. 

this will help you, op

https://blog.andertons.co.uk/learn/what-is-tube-amp-bias#:~:text=Run%20the%20valves%20too%20cold,tha....

I always ask the question "How does it sound?" I suggest if you’re having a bias issue, to recheck the bias a FEW times over a couple hour period and THEN call it good. The issue I have with running too cool is clarity at low volumes. I DON’T use valve amps for bass duty.. Period.. That changes how I bias also.

If the manufacture says 55-65ma per valve. 55 is the low side. 70% of that? No, not a good idea. I’ve dropped to 45-55 if I’m really blasting...
BUT it takes a whole 60 seconds to bias my rigs...

The rule use to be if you bias hot you run out of gas at the top, but mid level and LOW level playing sounded very warm and 3D the sound stage..

Nothing worse than yucky mids and rock and roll. Bias COOL will get you that... :-)