Scott. I think from what you say about the balance of the Mega Bass Voices in the Styles is possibly that the Styles were made long before they decided to add the Mega Voices. They have just inserted the Mega Bass Voices into the styles without considering that Mega Voices respond to velocity layers which is causing the balance problem you have noticed.
Its no good going the EQ Route because this will effect every other style and sound on the board.
Dreamer....That's possibly the same chap at Yamaha that left off the 3rd right voice Split from the 9000 & 9000Pro.
Graham UK