There should be a 'zone 2' selector button on the unit or remote of the Onkyo, and corresponding speaker connection terminals on the rear of the unit. Connect the input of the Niles unit to these zone 2 terminals. The Onyko should now allow for seperate volume level to the zone 2 input without affecting the zone 1 (Bose) volume. Set that where you need and then control your seperate zones from the Niles.
without knowing the exact model of reciever...
There should be a 'zone 2' selector button on the unit or remote of the Onkyo, and corresponding speaker connection terminals on the rear of the unit. Connect the input of the Niles unit to these zone 2 terminals. The Onyko should now allow for seperate volume level to the zone 2 input without affecting the zone 1 (Bose) volume. Set that where you need and then control your seperate zones from the Niles.
RG