Lac, welcome!
I have a set of M80's being driven by a Yamaha 6.1 HTR-5660 in a room that measures 20' x 16'.
First, my room is smaller than yours and I have no complaints. Out of the box, the 80's sound great. I will conceed that after extended placement tweaks, the soundstage is definitely better. You could argue all day about them being "fussy" to place, BUT it all depends on YOUR room.
Second, your Yamaha is more powerfull than mine. I have driven it hard for extended periods with no adverse effects. (It only gets slightly warm, and has never shut down). You should have no problem driving the 4 ohm load. BTW - My surrounds are 4 ohm as well.
Third, Axiom makes a good sub for the money. You should be very happy. The problem is that for about the same money, you can get a GREAT sub.
My vote is for the Epic 80 - you won't regret it, and you won't ever wonder "what if".
Shawn
