Hi Kay,
Large tower speakers with many drivers like the M80s often have a 4-ohm impedance. A 4-ohm load causes more current to flow through the output transistors so they run hotter, and it makes greater demands on the power supply of A/V receivers that have five or seven built-in amplifiers. Impedance has nothing to do with sound quality. It is an electrical characteristic.
Consequently, I recommend only Denon, Harman/Kardon and new NAD to drive the M80s. I'm still skeptical of the new Yamaha 1400 & 2400 because we don't have enough feedback yet. Many other brands will not drive the M80s without protection circuitry shut-down or severe current-limiting, which limits the output power of the receiver to a fraction of its usual output.
The M60s are an easy 8-ohm load that any A/V brand will drive with no problems. The M80s have a very similar tonal balance to the M60s, but somewhat greater deep bass extension and output. They will also play extremely loud in big rooms and have huge power handling (700 watts per channel or more).
Regards,
