I have a couple non scientific suggestions you may want to try

do the m80s match each other well? Try listening to 1 m80 and the center channel if you are not sure. it may be the m80s are off, and not the center channel.

check the connections between reciever and vp 150.

what other equipment are you using to power your setup

I am not sure, but it may have something to do with the fact the m80s are 4 ohms and the vp150 is 6 ohms. probably not though

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