If you are looking to add bass to your Sonos soundbar system, a subwoofer is of course the best choice to do so; and right now is a great time too as the Sub 4 and the Sub Mini models are both excellent deals (purchase links down below).
The Sub 4 employs 5-inch x 8-inch dual elliptical woofers that face inward for a force-cancelling effect, which is said to enhance bass response and also reduce enclosure resonance. For those not familiar, force cancelling works since the drivers are acoustically in phase as they face each other even though they are out of phase mechanically. With the help of this technology, Sonos claims that the Sub 4 can go down to 25 Hz. Although not specified, it is likely at -6dB (F6).
The Sub Mini works on a similar principle and it too claims to go down to 25 Hz even though it has a smaller woofer surface area with dual 6-inch drivers. Again this will be more like at -10 dB.
Both the subwoofer models are powered by Class D amplifiers and thus they should be quite power efficient too. Speaking of power, Sonos does not mention the specific RMS output wattage of these subwoofers although media and user reviews typically say they have plenty of sound.
The following Sonos soundbar models are compatible:
