There is only a few inches in height difference from the M3 on 36" stand to the M80s. I doubt that is the problem. Have you checked the binding straps on the back of each M80 are they both flush to the bind post?
And lastly there is quite a difference from the M3 to the M80s, the M80s are much more forward sounding than the M3s, can you desrcibe what the differences are that you are hearing?