Apparently this puppy has 2RCA input, so ideally you are looking for an active (self powered) sub with 8" or large driver, IMO, with the following hookups:
RCA L+R input
Crossover Knob
Gain knob.
RCA L+R Output (after having gone through crossover, send small signal back to soundbar).
So.... TV ---(full range signal)---> SUB ---(high passed signal)----> SB.