A couple of simple basics to check first. Be sure the binding post on the back of the M80s are tight and the jumpers (metal bars between the 2 sets of posts) are secure.

Be sure the polarity is consistent, with the positive terminal (red) on the receiver connected to the positive terminal (red) on the M80s, and the negative (black) is connected to the negative. Speakers wired out of phase can sound kinda hollow.

Jack

"People generally quarrel because they cannot argue." - G. K. Chesterton