The horizontal centers are for those who can't fit a regular speaker into their space.
BTW, the M5s are 6 Ohm. Look at the impedance curve (on the M5 bookshelf page). This is of no practical significance to your Onk and the advice I gave you still stands. Axiom specs them as 8 Ohms to avoid a ton of calls from prospects who don't know what an Ohm is and what it means.
Cross them at 80Hz, turn it up, your Onk will be fine. I have the Onkyo 838 and it has a power supply and amplification similar to yours. In 2-channel, driving M5s, it's practically no different than an ADA-1000-2. I know, because I've measured. I freakin' love my Onk and Visions for the deep discount they gave me on it.
