Bose systems have a bass module, not unlike a powered subwoofer, but Bose splits the frequencies differently and uses a wider range of low frequencies that emerges from that unit. That design allows Bose to handle the higher frequency audio with very small speakers. The black box handles the low frequencies from both channels and the low frequencies are omni directional, long wave, and not easily absorbed so the box can be placed most anywhere and it would seem that Beaver has found some good hiding places over the years.. That box also contains a power amp for that unit on most systems with a bass control knob to set the level. My unit is in a compartment under the couch. There is a second system in my coach and that same module is under a shelf in the rear closet for the bedroom system.
Later Ed