Yes. "Harshness" from the speakers like you are describing during those dynamic peaks is a sign your receiver is not capable of powering your speakers in your room. This in undoubtedly the cause. You would definitely benefit from an external amp. Since you run a VP180 and it will dip below 4 ohms at some frequencies I would highly recommend getting an external amp for the front 3 speakers (M60 and VP180). Does your receiver have pre-outs?

I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne