Importance of impedance matching cartridge with phono stage?

Certainly listen with no resistor at all first.

A whole lot of low-output MC’s like to be loaded 100 ohms or less in conjunction with many active MC phono stages.

This statement is false (and a very common myth). The reason for the 'loading' has nothing to do with the cartridge and everything to do with the preamp's ability to deal with RFI.

The correct term for the resistor is its a 'detuning' resistor. Here's why:http://www.hagtech.com/loading.html

As you can see (from the link), the peak is a good 30dB more than the signal itself; if your phono section isn't good with that sort of RFI at its input, it won't sound right and by detuning the tank circuit at the input of the phono section relieves the preamp of the RFI so it can perform properly.


But there's more- the detuning resistor forces the cartridge to do more work and that energy has to come from somewhere- which means that the cantilever becomes stiffer and less able to trace high frequencies.


You are far better off if your phono section can deal with the RFI. A side benefit of this is you will likely get less ticks and pops, since overload margin at RF frequencies plays a role as well (IOW many ticks and pops are caused by overload, not the LP itself).


Many phono preamp designers simply don't realize that a good phono section is more than enough gain, proper EQ and low noise. It also has to have good overload margin, be resistant to RFI and otherwise be stable (and FWIW feedback or passive EQ has nothing to do with it). 


If the preamp itself has options for loading its a good bet the designer hasn't thought this through.