The grid stopper resistor RGS blocks radio frequencies while allowing audio signals to pass. This keeps the amplifier from becoming a radio receiver and prevents the stage from breaking into parasitic oscillation.
Large values can be used to attenuate treble, but even if this is desirable there are much better ways of achieving it. In general there should be very little attenuation at 10kHz and substantial attenuation at 100kHz.
The frequency response for a given resistor value depends on the inter-electrode capacitances of the tube and the voltage gain that it achieves. The calculator computes the response at various frequencies based on these factors.
1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).
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