Cartridge Impedence

Can someone out there give me a 'in a nutshell' explanation of the importance (or lack of) for a cartridges impedence? Much obliged in advance and I hope this isn't too obtuse a question.
The impedance of the cartridge has nothing to do with its sound. The information is supplied to assist in setting the cartridge up properly, specifically this has to do with loading. The loading at the input of the phono section is sometimes about 10x the impedance of the cartridge, but this actually depends more on the phono cable and preamp itself, as there is more to it than this 10x rule of thumb.

There are other threads on loading that have covered this, in a nutshell the loading becomes important if the preamp is unstable with RF energy at its input and the cartridge is a low output moving coil.

Beyond that that value of the impedance might be mostly intellectual curiosity.

Now with high output moving magnet, its a different matter. Loading with these cartridges is more critical, and is, IME, never the stock 47K value often specified. Again, the 10x rule of thumb should be explored first.
In my experience the higher the loading the brighter the MC cart will sound. lower, less so.
From just listening, I prefer 100 ohms loading for my Benz Glider, and Dynavetor 17D3.
My thanks to both Atmasphere and Elizabeth for their responses, helped to clear things up for me nicely!

02-25-11: Atmasphere
The impedance of the cartridge has nothing to do with its sound.
this statement is not true. what happens is that the cartridge, tone arm cable and phono stage input form an RLC filter which can have a resonant frequency in the audible range:

the bottom line is this is that near the resonant frequency (where there is a peak in the frequency response), energy can get reflected into the phono stage resulting in distortion. my guess is that pops in the record that occur at high frequency are probably what is of most concern here. what the cartridge loading does is reduce, or dampen, signal energy at the resonant frequency so that you get fewer unpleasant frequency artifacts in the audible frequency range.

as to the comment by elizabeth about the sound being brigher for higher impedance loads; this statement makes sense. the motion of the stylus generates induces a current, so the higher the input impedance at the phono stage, the lower is the current required for the cartridge to achieve a detectable voltage level. so a higher impedance means that the cartridge is less heavily loaded (i.e. it doesn't have to work as hard to generate the voltage) which results in improved dynamic response. in other words, the cartridge sound brighter because frequencies with less energy (and therefore generate less current) can achieve sufficiently high voltage levels to be heard.

if you really want a good explanation of this, jonathan carr, who designs the lyra cartridges, has provides some really good explanations on this. look through some of the comments in the thread entitled "anyone using a lyra delos yet".
oops, my mistake - atmasphere was referring specifically to the intrinsic impedance of the cartridge and not the loading impedance at the phono stage. for a mc cartridge at least, the intrinsic impedance of the cartridge is relatively small (i don't have a mm cartridge, so i haven't given much thought to mm cartridge issues). but cartridge impedance does play some secondary role in the sound of the cartridge. keep in mind that some audiophiles make a huge deal over small differences, so for some that "secondary role" really is a big deal. there are two ways that intrinsic impedance is impacted: one if through the number of coil windings around the cores, and the second is through the impedance of the coil wire. i have a lyra delos cartridge, however, if you compare the lyra delos and the lyra kleos, the kleos has a lower intrinsic impedance compared to the delos. the kleos also has a smaller output voltage than the delos, which suggests to me that the kleos has fewer coil windings relative to the delos. that said, while i have not heard the kleos (and my only knowledge of comparisons between the two cartridges is from comments that i have read in this forum) i would underscore that the difference in intrinsic impedance is a secondary factor in any sound differences; but as also stated, that "secondary factor" might be a really big deal to some.