hmerchen,
Make sure the two gold straps that link the woofer section with the midrange/tweeters at the speaker connectors on the back of the M60s are electrically making contact. It's easy to dislodge them when you first connect speaker cable. Then check that each driver is operating by putting your ear close to each driver. Use music with lots of bass, high frequencies and vocals that will let you confirm that the tweeter, midrange and woofers are operating.
In most cases like this, it's the gold "bi-wiring" links that are not making electrical contact. I'm certain that all you are hearing are the woofers in your M60s, which would account for the muffled sound.
The M60s have significantly more midrange and higher-octave output, making them clearer and more detailed than the M40s tonal balance.
Regards,
