Hi John,
I am a MMF7 owner. I am not certain of the arm on your MMF9 but if it is similar to the MMF7's Pro-ject arm, which I think it is, then there is a definite limit to the cartridges that you can use, i.e. forget about any of the low compliance MC models because the effective mass of the arm is too low. I believe that the Music Halls can accept any tonearm that is based on the Linn design so that may be a way of getting to better cartridges. Now it is my understanding, (and I will defer here to TWL, DougDeacon, and others with more experience) that the biggest sonic difference will come from a table upgrade. That would also give you a way to change tonearms.
Or, as Jrd351 says you could just keep what you have and be happy.
As for me, I'm saving my pennies for a complete upgrade.