Diki,
I too didn't like that the bass inversion kept coming on with different music assistants. It took me a while to figure how to turn it off. Here's how to do it:
1) push menu
2) go to global and push enter
3) scroll down to Performance Hold and push enter
4) Scroll down to Bass Inversion and push enter
5) use the wheel and select "on" (off is how it comes from the factory)
6) Push Exit
7) MOST IMPROTANT STEP: Scroll down to the bottom in global mode and go to "save global" and push enter, and then use the wheel to select "yes" and push enter. Now you are done. First time I forgot to save the global and next time I turned on BK-9 the bass inversion hold was back to off!
Also, each time you turn on the BK-9 the bass inversion light will come on at power up. Just turn it off once and it won't come on anymore.
I hope this helps.