"If I just went with a 4.0 ft^3 sealed enclosure, would I still get that deep base rumble for movies?"
yeah, you would do ok to put each w15gti in a 4 cubic foot enclosure sealed.
an inuke 3000dsp will work for both amp and equalization. each channel will produce about 750 watts, which pushes the driver to about its limit in those enclosures.
yours would be an easy build and very capable system. be sure to build the cabinets solid with good bracing and make them air tight.
if you want to purchase flat packs, check out diysoundgroup.com. i think erich is having some 15" cutout baffles made for the 4.0 cubic foot enclosures. they cost a little over a hundred buck delivered and are a cinch to assemble.