Input Attenuator Value

Depends mainly on the source that is driving the attenuator. If solid state, 10k is ok.
If tube, you may need a much higher value, depending on the driving circuit.

The amplifier's input impedance is not the main issue, unless it is relatively low.
 
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They rate their preamp at +/- 1dB down to 2Hz into a 50k load.
So I would use a 50k pot, but no lower. A 100k pot would work
if a 50k is not available, but a 50k is better.

Such high impedance pots should be mounted inside the amplifier,
or at least in a shielded enclosure near it with short cables,
to minimize HF losses.
 
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