mc cartridge loading

Not worth the effort if you are talking about the difference between 100R and 90R.  However, when the manufacturer says "at least", that implies that higher resistances (less of a load on the cartridge) might sound even better than 100R. Whether you would think so, if you tried it, depends upon your personal taste, the reactance of your phono cable and phono stage input, even the topology of your phono stage.  MC phono cartridges are typically loaded down in order to tame a rise in response at very high frequencies that can occur when the aforementioned factors are in a certain alignment.  But in some cases, MC cartridges can sound surprisingly great into a standard 47K load resistance, a la that used for MM cartridges.

to lewm
Thanks for your response.
I might try reverting to 200 ohms, which is what is pre-installed in the phono amp, so involves removing the resistor I added in parallel without replacing it.

Right now, on two different turntables, I have set up a Koetsu Urushi, in one case, and an Ortofon MC7500, in the other, both looking at a 47K load.  Sounds really good, but not necessarily better than into, say, 1000R or even 100R.  (I do think I like them better into 1000R or 47K vs into 100R but would need more listening time to be sure.)  This is using an Atma-sphere MP1 preamplifier.

I'm with you lewn.....Most of my cartridges operate at 1000

The loading only needs attention if the phono section is sensitive to RFI at its input. The RFI is generated by the cartridge itself in part due to the afore-mentioned resonance that is set up by the inductance of the cartridge and the capacitance of the interconnect cable from the tone arm. If that is kept low and if the preamp is stable the loading will have very little effect.

What is happening here is Audio Technica is essentially stating that the preamp they use isn't stable. Just because that is so does not mean that it is that way with you. If you do hear a difference (usually an improvement) with loading, it suggests that the phono preamp is unstable at ultrasonic or RF frequencies.

Bottom line: loading is about the phono section and not the cartridge.