What would happen when you under Bias your tubes?

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)

 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

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

no

not if you want proper sound the amp maker intended

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.