The M80s have a more forward presentation, the mids and upper range are more pronounced than the M60 at lower volumes. Once you crank up the volume on the M60s they sound almost identical to the M80s, at least that was my experience with the M60/M80 when I had them both.
Others on here have run M80s with Onkyo and haven't had problems, just depends on how long and loud you listen. I think Axiom shys away from them just to cover themselves if someone has a problem, as Onkyo's reportedly have had power supply issues in the past.
You should check the audition area in the Water Cooler forum to see if anyone around you has the M80s for a good comparison.
