Hi Jhunt17,
Getting a separate multichannel power amplifier will usually get you a lot more real power output (from 200 to 400 watts per channel into 4 ohm loads) than an AV receiver is capable of, and for lots of enthusiasts with big rooms, large low-impedance tower speakers like the M80s and an appetite for very loud, clean playback levels, the separate multi-channel power amplifier will deliver the goods.
By contrast, an AV receiver's power output is typically measured with only a couple of channels running at full rated output while the others run at 1/8th power output whereas multichannel power amplifiers will generally deliver their full rated power output with all channels running.
It's really a matter of cooling. It's impossible to stuff five or seven high-power amplifiers with huge power supplies onto one receiver chassis and keep the thing running cool enough, so compromises have to be made. Lots of AV receivers won't even drive low-impedance (4 ohm) main speakers like the M80s without overheating or shutting down. The protection circuitry of those models do that to prevent the output sections from burning out.
