I found the same problem on a M6-R I just picked up used. Sometimes as you say the one osc. is brighter, sometimes seems to be louder. What you could do before you decide to open it up or send it to a shop, is this: watch the periods in the display to find which is the rogue voice.Then you'll have to put your patches in key REASGN mode (got the manual?) choose which notes you don't play often and when that voice comes up, play one of those notes. At least you can contain the problem for the meantime. I bet there is a trimpot for each osc. to control the input level on the circuit board, it's amatter of finding which one there's 2 per voice. Good luck.mn