Hi Colleen, welcome.
Ajax, Seabear and others have already given you sound (ha!) advice.
My opinion is that a good mid-level sub like the EP350 would be PLENTY for your needs. Furthermore, if you are primarily listening to music (rather than watching DVD's, etc.), you may not really notice an meaningful improvement depending upon what kind of music you prefer. I had M60's for about 8 months before adding a sub and found that, while I appreciate the sub very much for movies, only certain music that I listen to is noticably improved.
A good sub is a fine thing to have. And, whether it makes economic sense for YOU is a matter of your listening habits, your room and your budget. Enjoy the M60's, they are fine speakers.
So, what kind of music DO you listen to most often?
