Unless you plan on integrating your subs with some type of external crossover, preferably an active one, you will be throwing your money away by adding subs to your current system. If you are not planning on using an external crossover it is usually considered best to set the low pass filter on the sub approximately at the 6db down point of your main speaker which in your case would be somewhere below 17 Hz which is below the threshold of human hearing. With the exception of “chill” in the music you list, you are most likely not reproducing anything under 40 Hz and with chill you are probably not going much below 30 to 35 Hz so you are still above the the subs low pass filter setting. If you did set the low pass filter on the sub higher without a crossover you will very likely have major phasing issues in the overlapping frequency range between the sub and your mains making the bass in that region close to unlistenable.
People recommend two to four subs because multiple subs very much smooth out the peaks and nulls that can be created by low bass that room treatment can not do as well, not to mention it takes tremendous power to reproduce very low bass.
If you like to listen at loud volumes you would want a sub 12” or larger. Also plug any ports in the B & W’s if there are any.
This is a good read if you are new to subs.