Loading on low output moving coil cartridges is really not about anything at audio frequencies. Its about high frequency resonance, well outside the audio band unless you have a very high inductance cartridge and very high capacitance interconnect.
The reason is that the inductance of the cartridge and the capacitance of the cable form a tuned circuit, often in the Radio Frequency range. If the preamp has problems with bursts of RFI at its input, the loading is likely to be critical. Otherwise, it won't be.
The thing to note here is that with LOMC cartridges, loading is all about the preamp and nothing to do with the cartridge. If you think you hear effects from loading (more laid back high end, things like that) its an indication that your preamp has RFI issues. More likely this will be a bigger problem with solid state preamps, where RFI is more easily rectified. But design problems can cause tube phono sections to rectify (convert from RF to audio) RF as well.
When a manufacturer says a cartridge should be loaded at a really low impedance as suggested in this thread, its an indication that the preamp they use has RFI problems. One thing to consider is that the interconnect is a variable in this, and so the actual loading value that really does the trick is not cast in concrete either.
