From the SVS website:

Plants, your best friend's bottle of beer, and that 50 lb. bust of Beethoven are all very bad things to put on top of our cylinder subs by comparison [with their box subs].  Their  tops must be un-obstructed as their top-firing ports are protected by a relatively light-weight mesh grill.  If this matters to you, the box SVS's gain another point in their favor.