Yes. You can use the Line in/out connection of your BM7000 for the preamp out of your Onkyo, if you are talking about the plug that is covered with a connection plug. It`s just the connection preamp out/ poweramp in. I wouldn`t recommend to use the seven pin DIN plug labeled AUDIO AUX LINK or AUX...(?) This is an input for the preamp, same as all the inputs like TAPE, CD, PHONO...
Stefan