Hi Deano,
Check to see if your Denon's impedance setting is at the highest--8 ohms--not the lowest setting. Denons typically have no problem driving the 4-ohm M80 towers. If you set the Denon impedance to the lower setting, it will reduce power output by limiting the power supply voltage and current.
There might be a short from a strand of speaker cable.
The other possibility is that you are running the Denon at levels of loudness in a big room that outstrip its power output capabilities, in which case you may need to add an outboard power amplifier with several hundred watts per channel output.
Alan
