Damping or not on Sealed Subwoofer?

At the very least it should be lined. Depending on system Q it may need to be stuffed. That information, and for that matter the materials required, should have come with the kit.

A pound per cubic foot ?

That figure gets thrown about a lot, but it's meaningless. If the sub has a Q of 0.8 or less all you need to do is line it with a couple of inches of foam or polyester to damp modulation harmonics created by the motion of the cone. If higher than 0.8 the cab needs as much damping as required to bring it down to 0.8, if not lower. How much that is should have been determined by the designer of the cab when the prototype was measured. If the prototype was measured, that is.