I have the M80's as well and I found in my case that a they needed a little bit of toe-in and that changed things signifigantly.

As well a good point was made about set up for the receiver. Perhaps the speakers are set to small which could remove a lot of the bass from the mix. Check the menu if you have the onscreen display hooked up via your TV.

2xM80 VP180 2xQS8 2xM3 HSU STF3 LG 60PS11 Denon 3808 ATI 1506 LCR 2xATI 1502 Oppo BDP-83