I wouldn't sweat the 8 ohms vs 4 ohms. This is more of a regulatory issue. I drive my M80s and VP180 to loud volume without any issues with my denon 890 (given my room is smaller 20x11x9).
As Chris suggested, if you have the $$$ and the space for it, I would strongly recommend to upgrade to the VP180, that will give you a bigger improvement than M60s to M80, of course, if money is not an issue, go with the M80s as well (sorry, I had to say it
). As you probably read it many times if you did some research, the M80s are slightly more detailed and forward sounding in the midrange and as a better bass overall. This is what I noticed as well when I compared both at the Axiom factory.
Another suggestion for the center is that if you'll have often people sitting off-axis from the main listening position, you might consider a smaller vertical speaker (M2 or M22) as a center. If it's not the case, the VP180 is really good.