Hi seven,
In a general way, any A/V receiver designed for operation with 8-ohm speakers will always drive 6-ohm loads without any trouble. It is 4-ohm loads with the big increase in current flow that cause many receivers to go into protection mode or impose current-limiting circuits, which severely limit the output power of the receivers.
All manufacturers of receivers will be very conservative on this issue, as they don't want consumers driving pairs of speakers in parallel or very difficult impedances that might cause overheating and/or shutdown, although I'm surprised that H/K suggested caution with a 6-ohm load.
My experience with H/K (the AVR 525) is the same as kcarlile's. Despite the warning on the back panel (Use 8-ohm speakers), the 525 and other current H/K models will drive lower impedances--even 4-ohm loads--without overheating problems or protection-circuits kicking in.
The 525 will even drive the 4-ohm M80s with no problems, albeit in a modest-size living room (19 x 13 x 9 ft). On the basis of my own tests and those of several colleagues, H/K and Denon are the only brands of A/V receivers I currently recommend for the Axiom M80s. An engineer friend also tells me that the new NAD A/V receivers will drive a 4-ohm load without current-limiting or problems, though I don't have direct experience with those units.
Regards,
