What is the difference between MOTIF6, 7, and 8?

There is no difference in the operation of these units. The only difference between the three models is the number of keys.

MOTIF6 has a 61 note keyboard with Initial Touch/After Touch.

MOTIF7 has a 76 note keyboard with Initial Touch/After Touch.

MOTIF8 has an 88 note keyboard with Initial Touch/After Touch (Balanced Hammer Effect Keyboard).