Bryan, there's no need to be concerned with finding a receiver to "match" the M80s or other Axiom speakers. Clean amplifier power is usable universally with all speakers and the Axioms don't have any unusual requirements which would narrow your choices. The 3808 and 875 which you're now considering are available for nearly identical prices(using the factory refurb 875)and the decision might well come down to whether you want the Reon processor on the 875 for superior upscaling of DVDs and standard definition TV. If not, the 3808 has some other features that would probably make it the choice.
As to the sub, or subs, as Randy suggests, the EP500 would have slightly lower extension than a pair of EP350s, but the 350s would have the ability to smooth out the room response by placing them at the midpoints of the front and back walls or the sidewalls. As is true of any speakers, the two subs playing the same sound would result in a 3dB increase in level when they're widely separated(not within about a quarter wavelength at the bass frequencies involved).
