Most preamps can drive an array of FX types from pedals to rack units. So, for use in an amp FX loop, you can just set it however you want in the patch based on what sounds best to you (lower input impedances will give a high end roll offm, etc).

I would personally set it to the typical/default 1M most of the time and just control the sound with the POD FX settings rather than relying on some secondary setting like impedance when using the POD as an FX only processor. The setting is mostly useful for guitars going straight into the input to get the feel of that old low resistance vintage effect some people want to emulate and not really a big deal for an FX only application.

Anyway, bottom line is, your amp's preamp should have no problems with the POD's input and you do not have to have some magic setting so you can set it however it sounds the best for any arbitrary patch, with the general rule being higher (1M+) settings would be most transparent for FX.