How are they connected? PL to Aux-in? If so, I’d turn up the volume on the BS9000.
I think it’s also possible to connect from the BS’s AUX port… you just need to make sure to find an RCA to DIN cable with 4 RCA leads (2 input/2 output). However, as others have mentioned, the BS9000 needs to think it has speakers connected to it.
The auto-sense line-in works fine on my Balance (another Mozart speaker) – I have a turn table connected to it.