A shunt attenuator in theory will vary the input impedance based on the volume setting but is this something noticeable in "the real world"? I've read where it doesn't seem to be noticeable in practice and it's arguably the most transparent solution of the three attenuators (series, shunt, ladder). I'd be "driving" it from a modern DAC ...
You just design it for the minimum impedance the source can work well with.
For most solid state sources, a 10k series resistor should be fine.
For most solid state sources, a 10k series resistor should be fine.