UCD and stepped attenuator

[Beelde]

I want to insert a stepped attenuator in my ucd180 amps and I'm lost in impedance...

According to the data sheet, the input impedance of the modules is 100KOhms, yet, I found a message in which Bruno Putzeys advised a diyer to put a 10K pot and that t should work up to 20K.
I know that there are two ways to match the output impedance of the attenuator with the input impedance of the amp: to make it correspond to the input impedance (100K then) or to use a pot that has an impedance 10 times inferior to the input impedance following it (10K). So far, it matches the theory more or less...could anybody explain the respective merits of the different impedance values.
Having read that a 100k attenuator could start affecting the signal I would follow Bruno's recommendation yet, if I use a relay based attenuator such as dantimax kits, the target attenuation is reached only if the output impedance is set to 100K.
Could anybody be kind enough to explain the respective merits of such or such impedance values...

I have another question...
I want to use a DPDT switch to select betwen a rca and a XLR input. My original plan is to use a stereo stepped attenuator, half of which would be useless when the rca input is selected. I wonder if it's a sound idea...that is, would there be an impedance problem doing so?

Thanks!