Polk RM6750 Burning Smell

[Jeterocks]

I just recently got these speakers and hooked them up to a Marantz SR5002. They are only about a week old and when I turn the volume up to a high level (say around +5 or higher) for more than 5 minutes I can smell what smells to be a burning plastic/rubber smell.

Any ideas as to what this is? I'm not sure if it is just because they're new speakers or I'm actually harming them in some way. Thanks