IME, using two different subwoofer as stereo pair is not ideal and very hard to integrate well.
My suggestion is connect the SVS SB12-NSD to ZPre3 preamp and position the sub on the front.
Use the high level (speaker level) connect the Audio Source SW15 to the Crimson 275 output (so you can use small gauge speaker wires and don't need long interconnect cables) and move the sub for best bass response in the room, it could be in a corner, front wall, side wall or rear wall.