I also own the Epic 80/500 system and couldn’t be happier with it. The conventional wisdom the Joe_in_Ct cites is a great starting point. This reiterates what he said and expands on the reasoning behind it. http://www.axiomaudio.com/sub_setup.html
There are many other helpful articles where that one came from.http://www.axiomaudio.com/newsletter_index.html
One thing I always tell people is that this is your home theater so while following convention is a quick convenient way to get things up and running if you have the time and inclination experimenting may yield you even more impressive results.
For example for 5.1 and 7.1 listening my M80s are crossed over at 60Hz rather than 80Hz. I sit very close to them and like the added feel of the explosions hitting me from the direction they appear on the screen.
Also I’ve found that in my room the L/R surrounds sound better a little farther back than the Dolby standard, even when used in a 7.1 configuration.
Other people have there own experiences and preferences so I say play with your new toys. I’ve had mine for a year and a half and I’m still discovering new things about my Axioms that blow me away.