Assuming you have the right cable, if no speakers are connected to the BS9000 it will not send anything to the aux out on my MK3. I do not know why that is. However when I plugged a dummy earphone plug into the BS9000 the AUX out signal came.
I think any plug that fits will do, maybe just try plugging in a pair of headphones first.
Cheers
JK
BS9000, BS2300, BC2, BL2500, BL3, Bl2, BS1, BV8, BC4, A8