Greetings,
I have the Epic 50 system as well, along with a Yamaha htr5660 receiver, which is the equivalent of the rx-v 640. Take a look at the March 2003 Axiom newsletter in the archives about setting up a subwoofer. The article mentions setting the M50's initially to "large". On my Yamaha there is a setting in the subwoofer options for "both", which routes the bass to the main speakers and the subwoofer at the same time...you might want to try that as well.
b.