I basically make sure the sealed portion of the box is roughly equal to what the sub calls for. After that it probably helps to have a fairly efficient sub, but not necessary. It is important to keep the top lid of the box serviceable...so that you can replace drivers or adjust the throat size as needed. My first WO I used a pair of cheap american bass subs and it sounded ok...loud but not much SQ and there were signs of too much excursion...Quickly swapped out with AutoTek MM Subs and I was happy except for my odometer jumping around all over the place.