Passive "preamps" must make a compromise. On the one hand, the source components (especially those with tubes) want to see a high impedance, such as 100K ohms. For a reference, Reduction, Eros, and Quickie-with-PJCCS all work best with at least 50K, though we have found them sonically satisfactory at 15K minimum.
On the other hand, if the passive is to drive interconnects of any length, its impedance should be low. For a reference, Reduction, Eros, and Quickie-with-PJCCS all have an output impedance of 4K ohms, and we advise cables of 1 meter, or low-capacitance cables of 2 meters, maximum.
The most widely used value is a 10K potentiometer, which has a worst case source impedance of 2500 ohms at the -6dB setting, assuming the source components have a very low internal impedance. A 16K potentiometer would have a 4K worst-case source impedance.
Hope that helps.
