I own the ms-2 now, and have also owned a lot of grados (sr-225 , rs-2 , rs-1 , sr-200, ss-1 and hp-2).
My friend owns the sr-325 and have heard the sr-325 multiple times, so I can give you a good idea of what your getting and a quick sr-225 vs sr-325 vs ms-2 review.
The ms-2 and sr-225 sound kinda similar. Both are pretty neutral (the ms-2 is a bit more ), both have great slam, and both have good detail and bass. The difference is the ms-2 has more detail, better bass, and a slightly more depth sound stage (also it has a cooler casing lol). Both headphones are very good and you can't go wrong with either, but if you have the extra change go with the ms-2 because it is a pretty decent upgrade for the money. I actually prefer the ms-2 over the rs-1 (prob the ms-pro also but i haven't heard it) because of how it's so neutral (although powerful) and uncolored it is.
For people who are wondering about how the ms-2 vs the sr-325-
The sr-325 and ms-2 are the same headphone physically (ex ms-2 has no button and the text on the casing reads a little different). The only difference between the two material wise is that the ms-2's drivers are designed more for studio use where the sr-325 is designed for audiophiles. Because of this the ms-2 is very accurate and neutral and the sr-325 is rather bright (some may say harsh) but has a more forcefull slam ( which really only works in favor for rock). The detail of the two is about the same, if I was to pick which has more I'd prob say ms-2 but it pretty much sounded the same to me. The thing about the sr-325's detail is that because of how bright it is, it tends to be maybe a little too detail and causes fatigueness, where the ms-2 is calm and non fatiguing but still hyper-detailed (kinda reminded me of the hd650). Bass on the ms-2 was a little smoother but pretty much the same as well. Mids sounded the same (if you don't include the tone of the mids). Highs are a little smoother on the ms-2.
The problem I found with the sr-325 was that although it is just amazing with rock, it pretty much well... sucked with every thing else. A lot of jazz recordings were just to damn harsh to listen to, and classical just sounded to dry and bright. The ms-2 may not have the same power the sr-325 has on some rock recordings but I still feel the ms-2 is 2nd to best with rock, and sometimes better on some recordings (actually rock sounded a lot like the sr-225 just with more detail). Also the ms-2 works wonderfully with ALL music, actually there are a couple cd's (like the white stripes rbc cd, and yo-yo ma obrigado brazil) that I was just amazed with. I've had and heard lots and lots of good headphones( only a few have I not heard) but on certain recordings you just can't beat the ms-2. If the ps-1 sounds like that on every recording,then it is deff the best headphone in the world lol.
Any way the sr-225 is a good deal but like I said, if you can, get a ms-2.
rock on