The volume level from MY MM6 has suddenly dropped off. no idea why. Have checked all the cables and they are fine. Seems to be a decreased output from the board itself, but I don't know why. Help!

I had the same problem. This is how I fixed it:
1- Lay the MM6 upside down, remove the screws, open the back panel.
2- On your right side (the Volume PC board section), you will see component stand up (2 of them had 3 legs each, one of them is had more than 3 legs don't remember exactly) but 3 of them all had metal finish on their back. There are look like Power Transistor ( I do not have a schematic to be sure) but when these part get warm or hot (volume is only decrease when MM6 had been Turn-On for awhile), caused the volume to decreased.
3-Go to Radio Shack or other electronic stores, buy 3 HEAT SHRINK or you can make them your self (find- google to see what it look like, marterial...etc);
4- Use small screws and nuts, install these HEAT SHRINK into these 3 component then they will help these 3 components to stay cool all the time, therefore it will fixed the Volume decrease problem.
5- Yamaha should put HEAT SHRINK into these part when they designed.

I have had this problem too. Often just turning it off and restarting the keyboard fixes the problem, however a year ago this easy fix did not work and so I opened the machine up. I had to remove a few screws and turn the circuit board over. Sure enough, near one of the output jacks there was a solder joint that looked darker than all the other. I uses a solder gun to reheat the solder joint and all worked well again for another year, at which time I had to do it again. I wonder if others have been having this problem.