Hello Paul,
Yes, the resistor approximately sets the input impedance of the amp. There are also input capacitances that should be considered, but very few source components aren't going to be able to handle these.
If you have no resistor, then there is no path to ground for each grid of the 12AT7. When the tube begins to operate, electrons will hit the grids, and without a path to ground, the tube's operating point will shift all over the place (potentially damaging the tube). In some circuit designs where the designer is not very trusting of the level control, sometimes you will see an extra resistor from the output of the pot to ground (of a high value, usually 1M) in case the wiper fails.
The resistor won't attenuate the signal, but the output impedance of the preceding stage will be in series with the resistor at the input to the Stereomour. For example, if you decide that you want a pair of 10K resistors at the input (bad choice), and your source component has an output impedance of 10K, then you will lose half your incoming signal (6 dB).
Typically for a power amplifier, we use 249K resistors at the input. A preamp with a ~2K output impedance is driving such an amplifier, the loss from the output impedance of the preamp and input impedance of the amplifier will be 20Log(251/249), which is about -0.1dB (tough to hear).
-PB
