Resistive loading

High amount of loading (meaning a LOWER value resistor) will reduce the top end which means less "sparkle" and "air" on top and more perceived bass (because the tonal balance has shifted).  I personally don't like anything less than about 125 ohms with any of the cartridges I've heard because too much of the top end is lopped off, and the very reason for having a high end MC cartridge is defeated.  

While I like it pretty wide open, some loading may be necessary, as others have mentioned, to prevent RF interference.  I helped a friend with a Hovland preamp which suffered from noise and a brittle sound that turned out to be RF related (we did not know until we opened it up that the default loading was 100k ohms, a lower setting cured all of the problems). 

In the past, loading was needed to electrically tame the rising top end of cartridges as they approached their resonant frequency.  Most modern cartridges have their peak so high up, there is no need for loading to tame the peak, as far as frequency response is concerned, but loading may help with some phono stages that are not equipped to handle the ultrasonic peak overloading the phono stage.

in plain english

https://www.paradoxpulse.com/mc-cartridge-loading-101

more technical/mathematical explanations

http://www.hagtech.com/loading.html
https://www.mysonic.space/phono-cartridge-impedance-loading

Proper loading can become an even trickier issue if you are employing a step up transformer.  A good discussion of this can be found in a white paper put out by Rothwell:

http://www.rothwellaudioproducts.co.uk/html/mc_step-up_transformers_explai.html

The bottom line conclusion that they come up with is not to worry that much about adding loading when using a step up transformer, concentrate more on the turn ratio to get the right amount of gain.

My phono stage comes with loading on the primary side of the step up transformer, but given the relatively high value (low loading) and the really low source impedance of my cartridge, this loading has no effect on the sound.

Most modern cartridges have their peak so high up, there is no need for loading to tame the peak, as far as frequency response is concerned, but loading may help with some phono stages that are not equipped to handle the ultrasonic peak overloading the phono stage.

Overloading the phono section due to the resonant peak activating can often be heard as a tick or a pop, which might sound like its on the surface of the LP. This is why designing the input circuit to have high overload margins is important.

If a MM high output cartridge is used, the resonant peak can be in the upper portion of the audio band if the tone arm cable has capacitance that is too high (which will lower the resonant frequency). This is why phono cables are made with low capacitance.


Its worthy of note that loading the cartridge will inevitably cause its generator to do more work to drive that load. This will cause the cantilever to be harder to flex (that energy has to come from somewhere). Potentially this could lead to tracking issues, since the added stiffness will interact with the mechanical resonance of the arm with the cartridge installed.