Sarang, Ian pointed out the major reason why subs, for the most part, don't make use of non-parallel sides for the enclosure. The small "room" that the inside of the enclosure forms follows the same room mode rules as do our listening rooms. If the modes that would exist in that "room" are well above the frequencies that the sub actually plays, then reducing or spreading them out wouldn't be of benefit. For example, calculate the modes for a room say 1.2'x1.5'x1.8', or more conveniently, use a calculator such as this one
. Note that the resulting standing waves inside the subwoofer "room" are all in the mid-range and far above what frequencies are actually played.