MT and njoy, this might sound unlikely, but it actually was the problem for someone about two years ago: make sure that the woofers on the M60s are actually connected and it's not just the mids and tweeters operating. The connecting straps on the M60 binding posts have to be tight so that both the woofers(bottom post)and mids/tweeters(top post)are operating.

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Enjoy the music, not the equipment.