Most people seem absolutely thrilled with the M60's and I doubt you would see much of a difference until you wanted to play *extremely* loud. Upgrading the E60 package to include the VP150 instead of VP100 might be a good investment, though.
Depending upon your discount, I personally would still try very hard to still use an ES series receiver. They seem like good products and a good value for you. I'm surprised that the M80 load is an issue. If so, fixing that half of the equation would probably mean a move to separates or a significantly more expensive receiver from another manufacturer.
