Regardless of the bass extension spec, the 4" mid-woofer on that speaker would make a crossover of 80Hz or higher desirable to minimize bass distortion and allow for a cleaner mid-range. Just about any type of music would benefit, although obviously music with significant content below about 40Hz, such as pipe organ, would make a good sub especially essential.
A closed sub enclosure wouldn't necessarily be of advantage with respect to accuracy, anymore than the ported design of the speakers degrades their accuracy. Depending on budget and the extent of low bass content in the music played, anything from an EP350 to an EP800 could be suggested.
Be sure that your equipment has adequate bass management provisions to realize the full benefits of adding a sub.
