Don't replace the nut. Before taking extreme measures, it would be preferable to go back to the original type strings, (Worth clear mediums) which are almost identical to the M600s anyway.
Things to check:
Did you trim the strings? If the untrimmed portion of the string touches the speaking length, there's your buzz. No? Did you wind the strings going down from the hole in the tuner? If it goes up, you won't have the break angle necessary to keep the non-speaking length of the string from sounding and buzzing like a hornet.
Still haven't found it? Best to take it to a professional. You'll get it diagnosed and fixed for next to nothing or nothing at all. (Bring a set of strings with you.)
If you want to investigate further on your own first, see if a screw on a tuner has come loose, or the strap button. Something else (?) loose and rattling. Tighten up anything that should be tight. Washers rattle and will drive you mad.
Next, assuming it only buzzes on the open string, (un-fretted) and further assuming it did not buzz at all before restringing, the problem is most probably with the diameter of the string being larger than the previous A string. First be sure the string is the correct string. A larger diameter could cause the string to not seat itself in the nut slot. You should be easily able to tell if the string does not sit at the bottom of the slot. If it does, you're done. Have it checked out by someone who knows this stuff. If not, are you sure you installed the strings in the correct order? If you're sure you did, and you have the correct string in that position, that slot alone should be widened. Again, it's almost always best to go back to the original strings or go to your luthier, but if you feel you must make a repair, (sigh) you can do this with an emery board. Split it in half so you only have the fine side, with just the center cardboard on the other. Use this to sand one side of the slot very gently and gradually, being careful to not sand the bottom of the slot. You mustn't deepen it. Sand the side closer to the edge of the instrument (away from the other strings). Only sand until the string slides easily through the slot and NO FURTHER.
Still buzzing? Now you must have someone look at it. No matter what it is, it will be something very simple and not worth changing the nut.
Best of luck.
