The cutoff frequency and input/output Z of a filter design are dependent
on your requirements. Lower or higher values are a function of what you
are trying to accomplish.
In many instances the filter is designed to freq domain criteria, component
sizing, magnetic energy requirements, then the I/O Z levels are dealt with,
eg. their loading effects, or incorporated in the design loading effects as part
of the design.
Its a very broad topic with many fascinating twists and turns, design approaches.
Regards, Dana.