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.

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.

The 10X Rule is important mostly for achieving a flat frequency response within the audio frequency range (up to 20kHz), when two amplifying elements (e.g., preamplifier and amplifier, CDP and preamplifier line input) are in tandem; the input impedance of the driven stage ought to be AT LEAST 10X the value of the output Z of the preceding stage.  But this is only a rule of thumb. Even at a 10X ratio, the math tells us that there would be a small loss of signal voltage at frequencies nearing and beyond 20kHz.  In the case where the driver is a phono cartridge, which is also a transducer subject to inherent nonlinear behavior, the situation becomes more complex.  So, I agree that probably one should not go lower than a 10X ratio between the internal resistance of a phono cartridge and the input resistance of the phono stage, but a higher ratio can be better and usually is, at least where phono cartridges are concerned.  However, I have read on-line many times that a ratio lower than 10X is subjectively preferred by some of us, with some cartridges.  This seems to be true of the Denon DL103, for one example.  (I don't own one.)  

My preamplifier is an Atma-sphere MP1, and now that I have set it up so as to be able to switch easily between 100R, 1000R, and 47K, I find that I most of the time prefer 47K.  This gives a more "open" and richer treble, for want of a better vocabulary to describe it, but the difference between 47K and 1000R is subtle.

One of the most commonly used phono input tubes is the 12AX7 which has a very large Miller capacitance, given its high mu factor, albeit nowhere near to 10,000pF!  How did AcousTech manage to "achieve" that?