moving coil cartridge loading

Dave, firstly welcome to the forum.

As to the difficulties to which you refer, I would suggest several possible things to check/try before becoming overly concerned with tweaking the cartridge loading (provided you have it set to the manufacturer's recommendation)

Since no cartridge works in isolation, some indication of your partnering equipment might make it easier to suggest some possibilities.

Things worth checking are VTF (too much will make it sound rather "sat on").

VTA , arm "tail up" will accentuate upper frequencies, arm "tail down" the opposite.

Mat, felt can be a little characterless, rubber, bass heavy and a trifle "dead" sounding, acrylic, brighter and harder.

Cartridge "burn in" - new cartridges can take up to 100 hrs of play to fully develop their final character.

Clean cartridge pins and clips, clean RCAs through to phono stage, both on I/Cs and those fixed to equipment - CAIG deoxit is good for this - can be surprising the difference this makes.  

A friend who recently moved house and setup his system again, had an app 3dB channel imbalance when I first heard it in its new location - he had previously mentioned this to me whilst at his old residence though I had neither heard or investigated it.

A quick clean of several terminals and I/C plugs and the speaker connections (amp and speaker) - completely eliminated this issue, naturally overall SQ was also improved.

Basic cable housekeeping, keeping interconnects away from power cords, phono stages away from components having large transformers such as amps and CD players and standalone power supplies for other equipment.  Keeping the TT away from the above sources of EMI.   Where cables must be in proximity, ensure that signal cables cross power carrying cables at as close to a right angle as possible.

Proper cartridge alignment is also of prime importance in extracting the best from any cartridge.

For all I know you have already done all these things, if you have and are still unsatisfied, then load the cartridge as per the manufacturer's recommendation for a starting point and tweak that up and down until you are happy with the results or give up on the cartridge.

Capacitance is not of particular consequence with MC cartridges, so concentrate on impedance loading using either the settings of your phono stage or loading plugs of your own making in whatever values you may desire.

Remember that made up plugs inserted into a "T" adapter on the input to the phono stage will run in parallel with the inbuilt loading impedance, so a calculation is necessary to choose the correct resistor to give the required load.