That's a fairly good sized room. If you're 70/30 movies to music, I'd say get the EP500 first. IMHO, I think it would make the biggest immediate difference with the movies. However, either the M80s or M60s will make an equally big difference in the sound of your system.
I don't think you'd be "better" with M60s. But, with an EP500 in the mix, I do believe they'd do fine in your room, and you wouldn't feel like you sacrificed anything by getting them. On the other hand, if you have the budget and the desire, by all means get the M80s.
Just my $.02.
