The signals inside the soundbar amplifier probably have no deep low frequencies. You must add a low frequencies mixing amplifier at the soundbar inputs to mix together the stereo into mono and have a suitable lowpass filter to feed the sub amplifier.
Does the TV or other signal source produce any deep low frequencies? My stereo speakers produce good bass but I needed to add a bass-boost circuit to the amplifier for my TV.
I added an enclosed two-way speaker to my clock radio but it produced no bass until I modified the circuit with much higher value coupling capacitors, now it sounds like a hifi with wideband frequencies.