Possibly another thing the manufacturer factors in when stipulating minimum impedance is whether this might be for a head amp like you are using or an SUT. I have nothing to back up what follows being solely an SUT guy but I would be quite surprised if that choice mightn't affect the performance of the cartridge in quite subtle ways across the frequency spectrum and thereby adjust optimal impedance (which is itself a fudge of a figure)? Perhaps the figure given by particular manufacturers is aimed more at one type of setup than another, or maybe its just an average given as a general, hopefully helpful, indicator?
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