The most common cause of blown speakers is clipping. Your speakers may be rated at way more (2x,3x) power than your amp and you can still easily blow them.
If you set you volume to high when playing content with high dynamic range typically action movies, the spikes will easily exceed the limit of the output stage and cause it to clip. When the signal clips, you get DC current. DC is fatal to speakers. An amplifier of say 80 watts can easily fry a 400 watt M80 if it clips excessively.
Because you say you have blown many speakers, you are setting the volume too high. With the sound dynamics movies produce today, if your receiver is set to full dynamic mode (i.e. not night, evening etc...), you should really not be setting the volume more than about 1/3 (just a guess). Your Denon has Audyssey, try enabling dynamic volume and set it to evening mode, which should tame the spikes so you can do a general volume increase.
Or you can be a maniac like me and get an ADA1000 or other good amp to drive the fronts and even center. I have a Denon 4311CI and I use 3 channel ADA1000 my M80s and VP150.
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