I'm in agreement with Cropwell's interpretation. There's any engineering "wet thumb" rule that you design the input impedance at least 10 times the source impedance in order to not substantially influence the input. So, 10k input pot, 100k input impedance - rule is met. (ignoring balanced input design etc).

Don't get hung up on impedance over resistance.

And Nemo - Because I'm in a good mood I'm going to agree with you too - at least re. the circuit outside of the box being that that the user has to add.