To preface my *humble* opinion, I have gone from SR80is to the 225is, then the MS1is, RS1is, and have finally settled on the HF2s.
Even after listening to RS1is and going back to the 60is, they still sound good. No one should kid themselves- the 60s ain't no slouch!
The MS1is are a different headphone from the 60is (or any other Grado, really). Personally, I found them to offer superb value for $110. The 225is I had at the time were better, but I wouldn't consider it a whole step in the ways of an improvement. I was so torn between the two 'phones that I decided to do something extreme... I went straight for the RS1is. I was sick of incremental upgrades and similarity between the lower-end Grados, and going straight to the top cured my syndrome... except for the fact that I paid $813 for them. I started to have buyer's remorse because spending two years allowance on headphones seemed like too much (I so should have gotten 'em off the FS forum). Thankfully, a very close friend of mine came to my aid and saved the day: he traded me his iBasso D10 and HF2s for my RS1is.
I don't regret the trade to this day and I don't feel the slightest inclination to upgrade my headphones. I love to HF2s to the stars and really can't say enough good things about them.
My recommendation for you would be to do one of the following:
Upgrade to the SR225is and be happily wed. While they aren't better than the 325is(s), they certainly offer the best value in the Prestige line (I have no hesitation in saying that they do offer 80% of the RS1is at less than 1/3 the price).
If you get the MS1is, my primary concern is that you either won't like the Alessandro sound, or won't find the upgrade significant enough. In your case, I'd stick to the safety of my former recommendation.
Finally, if you want to be a big-spender, you can go straight for the big guns. You will save money as Uncle Erik said, but I think taking at least one step in between will help you develop a better appreciation for the upgrade.
Cheers!