You could connect it to the left and right channels. Without know what your passive sub looks like, I'd suggest that you should at least be able to run a separate pair of cables from the left and right binding posts. If the passive sub happened to have speaker inputs and outputs, you might also put it in series with the mains (run speaker wire from the 1070 to the sub and from the sub to the speakers). In either case, you'd need to set the 1070's mains to large and disable the subwoofer under the speaker setup menu. Do you have any details on the sub or the HTiB set it came from?