Check to see if you got the standard M80s with one set of speaker terminals on the back of each speaker, or if you went for the bi-wire version, with two pairs of speaker terminals connected by gold straps on each speaker. If the latter, make sure the straps are tight under the terminals and that the woofers are working. Set the receiver to the Stereo mode.
As troyd pointed out, if you're using Audyssey, up the front left and right levels to 0 dB in the menu.
Next, how big a room do you have? Length, width, height? If it's a giant room, you made need the subwoofer to get powerful bass. In a very large room, the M80s may need subwoofer assistance.
You may have placed the speakers (or your listening seat) in an area of bass cancellation. Just moving the speakers or your listening chair a foot or two may make all the difference. The closer you move the M80s to a rear wall, the greater the bass output; the farther out from the wall, the less the bass output.
Have you touched 9-volt battery terminals to the positive and negative speaker leads for each speaker to make sure the woofers on each M80 are in fact working? When you touch the positive and negative terminals to the battery, there should be a click and the woofer cone(s) will move inward or outward. You should see them move. This won't harm the speakers.
