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According to this article excerpt there is a risk of increased crossover dist. when lowering the screen grid voltage.
the screen voltage can affect the maximum allowable bias current, but it is dependent on the plate voltage and the class of operation. If you take a "standard" guitar amp and lower the screen voltage, you can raise the grid voltage (less negative) and bring the bias current right back up to where it was at the original screen voltage without overdissipating the plates at idle or under signal. In this case, since it takes a smaller negative grid voltage to attain the same plate current (because of the lower screen voltage), you will lose headroom. If you don't raise the grid voltage, you will simply have the amp biased more towards class B, with the resultant increase in crossover distortion."
See the full article here
Biasing
the screen voltage can affect the maximum allowable bias current, but it is dependent on the plate voltage and the class of operation. If you take a "standard" guitar amp and lower the screen voltage, you can raise the grid voltage (less negative) and bring the bias current right back up to where it was at the original screen voltage without overdissipating the plates at idle or under signal. In this case, since it takes a smaller negative grid voltage to attain the same plate current (because of the lower screen voltage), you will lose headroom. If you don't raise the grid voltage, you will simply have the amp biased more towards class B, with the resultant increase in crossover distortion."
See the full article here
Biasing
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