Just saw your second post. A sub would be more noticeable with the M60 than an upgrade to the M80. It will make a HUGE difference for movies where you get significant output below 30 Hz.
With music, unless you listen to something with significant output below 30 Hz (like pipe organ) you will not be able to tell the difference between an M80 with and without a sub. I did that test in my room several times.
Here is something to consider. I bought my system mostly for music, but opted to go for a 5.1 setup. When I put my first music tracks on, I got a giant sized grin. When I put my first movie on, I think it was Lord of the Rings, my jaw hit the floor!
Given a choice, I would invest in the sub now and add a center and surrounds for movies (and gaming) as your budget allows.
