step up transformer?

I know a couple builders who are big fans of Jensen SUT. The Jensen MC SUT use to be in your price range (don't know about these days, particularly with the weak dollar vs. the pound).

Rothwell, another English company also makes MC step ups that have a good reputation.

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.

I think you would hear a substantial difference only if you went with much more loading (LOWER value resistors for loading) than your phonostage allows. If you tried 30 ohm or 60 ohms or any other such values, you should expect to hear more of a change. The high end would be not as prominent (good or bad, depending on what you want), the bass would become more prominent, and a sense of "airiness" would decrease. At values such as 30 ohms or less, you will also be losing a substantial part of the signal to resistive loss, so volume would be lower too.

My own personal preference is to run most cartridges fairly wide open (high value resistor) because I value the sound of an open top end. If I think the sound is a touch too bright or sibilant, I adjust for this by both trying a little more loading (lower value resistor) or I change my cartridge setup by lowering the vertical tracking angle. To some extent, I treat vertical tracking angle and cartridge loading as interactive variables and experiment with both to get the right treble balance.