cabling a subwoofer to a NAD C356 integrated amp

I have the NAD integrated amp with A & B speaker terminals. I have a Gallo sub and Focal speakers. I'd like to have the Focals connected directly to the NAD, not through the Gallo. I'm wondering if the Gallo can be driven by the B speakers so I have A & B speakers driving together only for the B set I have my cables set for both right and left positive to go to positive in on Gallo, same for left. The gallo will not have any speakers attached, and I can turn it on and off at will. Any downsides to this?