It doesn't necessarily have to come from a subwoofer. Dolby specifically states that the LFO channel does not have to correspond to a physical speaker. The LFO channel only exists to ease retrofitting a subwoofer into old cinemas (in that case they would simply wire the LFO to a subwoofer and be done). But any other channel can contain bass information and in practice a new cinema will use a bass management system to route bass from all channels into their subwoofers. In theory a cinema could also have no subwoofers at all and send all bass to the mains and drop the LFO. It all works.