[Mike P.]

6 cubic feet sealed for 2 subs will work but you'll get less output in the 20 hz range compared to a ported box. They'll handle 600 watts with Xmax being reached around 18 hz and spl being around 106 db at 20 hz, plus the effects of room gain.
Doubling the size to 12 cubic feet, modelling shows less power to reach Xmax with minimal effect of spl and low end output, compared to 6 cubic feet.