For a LOT of MC cartridges, there is not much sonic difference between 100 ohms and 47,000 ohm (47k) because 100 ohms is, relatively speaking, close to being wide open; this is particularly the case with low source impedance cartridges. The difference you might hear between 100 ohms and 47k ohms would primarily be in the sense of "air" or openness on the top end (some would say "sparkle" to the sound, or a slightly greater sibilance on things like a cymbal or snare drum hit. You may also sense greater bass with the 100 ohm setting. But, given the choices your phonostage offers, many cartridges will show only subtle differences. Bigger sonic changes can be expected if your stage offered options like 30 ohms or 60 ohms.
For most cartridges, 100 ohms and upward work just fine. I would only be concerned about adding more loading (LOWER value) if the sound is unduly bright or sibilant. That would mean soldering such lower value resistor between the hot and ground tab on the inside of the input jack or making a loading plug that does the same thing, or replacing the loading resistor inside of the phonostge.
