Hi Dennis,
The speakers differ in the quantity and quality of very sophisticated components in their dividing networks and BAM.
Both of these models offer a significant sonic upgrade over the Millennium. The MX offers perhaps an additional 10% improvment in sound over the MM but you will really want to have a superb system for them to be at their best. The MMs are a bit more forgiving. Please have a look at the MX page on the main page of www.merlinmusic.com for a description of the sonic enhancements.
Regards,
Bobby@merlin