Hi agentfox,
It's not a design choice to make a speaker 4 ohms--see my post in the technical questions thread; rather, it results from running six drivers in parallel. And you are correct, most AV receivers have specific warnings about running 4-ohm loads because with seven amplifiers on the same chassis and, typically, one power supply for all of them, the latter can't deliver enough current to 4-ohm main speakers without overheating, etc.
However, certain brands we've tested at the plant have sufficiently robust power supplies that they will drive the M80s without problems (Sherwood Newcastle, Denon, H/K). You can try your Pioneer, but set it to the 8-ohm setting, otherwise it will automatically limit current and the power into a 4-ohm load. It will either drive them or it won't. You won't damage your Pioneer.
Regards,
