Mitch2 6-17-2016 4:59 am EDTThis is a good point that warrants additional emphasis, IMO. If the component that is driving a resistive-based passive preamp has an output impedance that is essentially constant over the frequency range, and if the component that is being driven by the passive preamp has an input impedance that is also essentially constant over the frequency range, then what might otherwise be an impedance incompatibility will simply result in a slight reduction in gain (putting cable effects aside, which are a separate issue, and assuming that the mismatch is not so extreme that the source cannot comfortably supply the necessary current).
It is not just about gain but I believe also about ... frequency variations with impedance.
But if for example the source component has a coupling capacitor at its output, resulting in a substantial rise in output impedance at deep bass frequencies, or if the output impedance varies significantly in other parts of the spectrum for whatever reason, using that same passive preamp may result (depending on the specific numbers that are involved) in objectionable irregularities in frequency response and/or phase response.
Perhaps that factor, differences in how source component output impedances vary as a function of frequency, is one contributor to the divergence of opinion about passives.
Regards,
-- Al